A Minor Kidroduction

Sometimes a work begins by showing us the main character as a child. It can also show us an important supporting character, but it's usually the main character. These kinds of introductions are barely longer than a few minutes at most, and they seldom affect the plot in any way other than being a way to highlight some basic character motivation, or perhaps provide the origin of some Catch Phrase. After the introduction is done, we are transferred to the present day (often via Age Cut) where the main character is grown up (and played by the featured actor or actress).

May show them playing with a Weapon Jr

Anime and Manga

 * Jo Jos Bizarre Adventure starts off with Jonathon Joestar and Dio Brando meeting as grade-school boys. Just a couple chapters later and we see them again as young adults.
 * Tekken the Motion Picture begins with a flashback with Kazuya Mishima and Jun Kazama as kids. This scene is very important for the plot later in the movie.
 * Code Geass starts with a little kid version of Lelouch and Suzaku surrounded by smoking debris, implied to be the result of a Britainnian attack. Lelouch vows to oppose Britannia for as long as he lives. Cue opening song.
 * Nightmare of Nunnally begins similarly, showing Nunnally and Lelouch at some point not long after their exile, with Lelouch promising to protect Nunnally.
 * Magical Girl Lyrical Nanoha StrikerS begins with Subaru as a kid being saved by Nanoha, motivating her to join the Time-Space Administration Bureau.
 * The first chapter of Letter Bee shows a seven-year-old Lag Seeing being delivered to Cambel Litmus, establishing his goals of finding his mother and meeting Gauche again. By the end of the chapter, he's five years older, and ready to set out to become a Letter Bee.
 * The title sequence of the Sailor Moon SuperS movie features the five main Sailor Soldiers as children.
 * Fullmetal Alchemist starts out with a page of ten-year-old Ed, right after his failed human transmutation.
 * The 2003 anime version likewise begins with said failed transmutation; two episodes later, we get the whole story.
 * The first chapter of the One Piece manga does this with Luffy. (The anime however changes it into a flashback episode just after Zoro joins the crew.)
 * Afro Samurai gives us one of these, introducing us to kid afro and providing him his motivation for the rest of the gore-fest to come.
 * Love Hina. It features two of the protagonists; later it has a bit of a retcon that adds a third one.
 * The first Fatal Fury OVA starts off with Terry and Andy (and Lily) as kids, and shows us the source of their motivation for the remainder of the cartoon.
 * Special A begins with kid Hikari practicing martial arts with her father, and then kid Kei comes and defeats her easily, realeasing plot.
 * Bakuman｡ begins with Mashiro Moritaka getting a visit from his uncle Mashiro "Kawaguchi Taro" Nobuhiro (who is dead by the time the main story begins), who praises him for his artwork and suggests that he will be an artist when he grows up. Moritaka then notes that he wasn't referring to him becoming a Manga Ka.
 * Hayate the Combat Butler begins with a flashback to Hayate's childhood where Santa tells Hayate that he wouldn't ever bring him a present, and provides one of the reasons why Hayate is so strong-willed.
 * Arashi no Yoru Ni begins with Mei (still a kid) being forced to run away when his mother is mauled to death by wolves. A kid is actually a baby goat.

Film

 * Forrest Gump starts off with fifteen minutes of scenes of Forrest's childhood.
 * Miss Congeniality begins showing that Gracie to be quite "masculine", even as a girl. She physically defends a boy from bullies, but he thanks her by telling her to get away from his face.
 * Ever After begins this way,with Drew Barrymore's character as a little tomboy who quickly on loses her father to heart attack,then the main plot takes place ten years later
 * Sister Act: Deloris Van Cartier shows her disrespect against Roman Catholicism and Roman Catholic convents by naming the apostles John, Paul, George, and Ringo. The nun says that Deloris will be doomed to a life of sin if she doesn't change her ways. Cue the grown-up Deloris being involved in a relationship with a married mob boss in Reno.
 * Shallow Hal begins with the death of Hal's father while deliriously preaching about titties.
 * Tenacious D and the Pick of Destiny has young JB at the start (different actor than young Hal).
 * Space Jam
 * Amélie
 * The 2004 version of Final Cut (maybe the other one too, I wouldn't know) does this.
 * The 2009 Star Trek starts with Kirk's birth and gives childhood scenes for both Kirk and Spock. The scene with Kirk's birth is important to the plot; the other scenes are not, but they do help establish what kind of people the two characters are.
 * Pirates of the Caribbean begins with a shipwreck scene where Elizabeth and Will are both children.
 * Splash begins when the main characters were kids.
 * Conan the Barbarian began when Conan was just a little boy being taught about the riddle of steel. However this would be an aversion since the movie shows Conan in various stages of his upbringing until he reaches manhood.
 * The 2011 film goes a step further, showing him in the womb--just before his mother is stabbed in the stomach, necessitating a battlefield C-Section by his father.
 * Comedy film Kinky Boots opens with drag queen Lola dressing up as a girl when he was a little boy. After his brief introduction as a child, he spends the rest of the movie as an adult.
 * Up opens with Carl and Ellie meeting as little kids, and it does set up the rest of the movie... by showing how important they were to each other, and their united dream of seeing the wilds of South America.
 * Bolt starts up with the titular dog as a puppy getting adopted - though whether it's in the show or not is left unclear.
 * Anger Management starts with Adam Sandler's character as a 12-year-old.
 * The Eddie Murphy version of Doctor Dolittle.
 * Twister begins with the lead heroine losing her father in a traumatic way early in life.
 * Contact begins in the same way.
 * Almost Famous begins with the protagonist as a 10-year-old before fading to him as a teen.
 * Art School Confidential opens with Jerome as a kid.
 * Used in some biopics, such as Man on the Moon and The People vs. Larry Flint.
 * Indiana Jones and The Last Crusade starts off with an extended sequence featuring Indiana as a teenage Boy Scout. It ends when he loses the Cross of Coronado, and there's a cut to the adult Indiana on a ship trying to recover the Cross.
 * Treasure Planet starts with Jim's mother telling a bed-time story about Captain Flint and the planet where he hid his treasure.
 * National Treasure starts with the main character being given a clue by his grandfather, 30 years before the rest of the movie.
 * The Princess and The Frog opens with Tiana and Charlotte as little girls, hearing the story of... well, guess.
 * Cloudy With a Chance of Meatballs opens with eight-year-old Flint showing off one of his first inventions, and later receiving his trademark labcoat from his mom.
 * During Watchmen's opening sequence, we see a very young Rorschach staring balefully at his prostitute mother's clients.
 * And we see a young Laurie walking in on her mother arguing with her stepfather.
 * Pearl Harbor begins with Rafe and Danny as young boys, their love of flight, and Rafe protects Danny, setting up the central action of the film.
 * Firefighting movie Backdraft begins with the main characters, Stephen 'Bull' McCaffery and Brian McCaffery, as kids.
 * Fireproof opens with the heroine having a Mother, I Want To Marry My Father conversation in voiceover.
 * Mo Better Blues opens with Bleek being forced to practice his music instead of playing with friends. Interestingly,, played by the same actor as young Bleek, goes through the same thing as Bleek-only, this time he is allowed to play with them.
 * Carrie would have started out like this had the special effects necessary for the scene worked properly. They didn't, and so the scene was scrapped. The remake, thanks to the advance of CGI, was able to include this scene, albeit not at the very beginning.
 * The Rage: Carrie 2, the sequel to the above film, started with a scene of Rachel's mother trying to "protect her from demons" before Rachel is taken by social services and her mother is taken to a mental hospital.
 * The Lion King II, unlike the original, only briefly shows Kiara as a kid on the day she meets Kovu.
 * Tron Legacy opens up with Sam Flynn as a child, talking with his father about the plot of the original Tron the night before he mysteriously disappears. Also notable for being the only real-world scene where CGI is used to make Jeff Bridges look younger.
 * Goodfellas starts showing how Henry Hill enters the mafia world during his childhood. It age cuts to Ray Liotta after a few minutes.
 * Cinderella briefly shows the titular heroine as a little girl during the film's prologue.
 * At the very beginning of Sleeping Beauty, Prince Phillip for some reason, is a toddler at the same time Princess Aurora is a baby.
 * In those days, it wouldn't be all that unusual for a girl to marry a boy several years older than she (think Romeo and Juliet, for example).
 * Ariel was briefly shown as a little girl at the very beginning of The Little Mermaid III: Ariel's beginning.
 * At the very beginning of Dinosaur, Zini is actually shown as a baby lemur alongside with a pregnant Plio during the scene where Aladar's egg is shown landing in a small bush on Lemur Island and immediately hatching out of said egg.
 * At the very beginning of The Emperors New Groove, Kuzco is briefly shown as a toddler just right after he is introduced in his llama form before cutting to his adult human form prior to his transformation.
 * Atlantis the Lost Empire begins with a young Kida crying for her mother, the former Queen of Atlantis as she is sacrificed by the Crystal to save her kingdom from a tidal wave. This fails, and as a result the Queen dies and Atlantis ends up underwater.
 * Milo was also briefly shown as a child in both an old photograph of his as well as on an old film reel just right after the titles appear.
 * Andy Davis at the very beginning of Toy Story 3.
 * Most of the X-Men movies start this way:
 * The first movie starts with Magneto as a boy in a concentration camp.
 * X-Men: The Last Stand starts with a younger Professor X and Magneto visiting Jean Grey as a child.
 * X-Men Origins: Wolverine starts with Wolverine and Sabretooth as children.
 * X-Men: First Class starts with Charles Xavier and Mystique as children, and recaps young Magneto in the concentration camp.
 * Tangled begins with Rapunzel as a baby, then briefly shows her as a toddler and a young child before starting the main plot on her 18th birthday. These scenes are relevant, though, as they show Mother Gothel kidnapping Rapunzel from the castle and Rapunzel developing her obsession with the lanterns, both of which drive the plot throughout the entire movie.
 * Thor begins with a young Thor and a young Loki being told the story of Asgard's fight with the Frost Giants by Odin, with both Thor and Loki showing elements of their personalities: Thor eagerly promising to wipe out the Frost Giants once he's old enough and Loki eagerly seeking their father's approval.
 * Some of the more recent trailers for the upcoming film Brave show a young Merida playing with her father King Fergus' bows and arrows while her mother Queen Elinor (pregnant with three baby boys) looks on, in what would appear to be a prologue in the actual film.

Literature

 * Discouraged and mocked as "The Long Runway" in How Not to Write A Novel.
 * If it counts when the main character is already a kid in the present, the first Harry Potter book starts with Harry being left to the Dursleys as a Door Stop Baby. The second chapter skips forward ten years to the preteen Harry we'll follow for the rest of the novel.

Live Action TV

 * Each episode of Pushing Daisies typically starts with a look into the childhood of one of the characters. For most of the first season it was Ned, but later they added flashbacks of Chuck, Olive, Emerson, and even, in the last episode, Lily and Vivian.
 * A handful of Lost episodes have a childhood flashback in the first act, followed by a later-life story in the remaining flashbacks. "White Rabbit" begins with a scene from Jack's childhood, "He's Our You" with Sayid's. "White Rabbit" actually keeps the child story going for a few flashbacks, probably because it shows Jack's relationship with his dad, who is for the rest of the episode.
 * Nearly every episode of Psych starts with a "Young Shawn" sequence. One episode even began with a "Young Juliet" sequence.
 * The pilot episode of Supernatural begins with Dean as a kid and Sam as a baby, on the night that their mother was killed.
 * The Lone Gunmen frequently used this.
 * Amy Pond is introduced this way in Doctor Who, and boy, does it have a lasting impact on her life!
 * The first episode of Dharma and Greg opened on a subway with both characters with their parents just happening to be there at the same time, though they didn't actually meet.
 * One episode of Firefly begins with a flashback to when Simon and River were children, mainly showing how close they were and how River was a playful, imaginative girl before her brain was hacked up. There are several other flashbacks, mainly dealing with Simon trying to rescue River, but none have them as children.
 * A few episodes of Frasier open with Frasier and Niles as children, generally to establish that whatever behavior will drive the plot of the episode has been going on since childhood.
 * Scenes between young Jaron and Younger Sydney (the latter still played by Patrick Bauchau) were close to a Once an Episode feature on The Pretender.

Music

 * Sound Horizon's Moira begins by showing the twins of fate as children. In the span of one song they turn from kids to adults.

Theatre

 * The first scene of the musical On Your Toes shows Phil Dolan III as a child doing a vaudeville act ("Two-a-Day for Keith") with his parents, Phil II and Lil. The rest of the play takes place 15 years later.
 * The 25th Annual Putnam County Spelling Bee opens with one of the announcers for the titular bee winning the 3rd Annual Putnam County Spelling Bee.
 * In Les Misérables, we are introduced to Cosette and Eponine when they are children. Cosette gets her song "Castle on a Cloud", to establish how she's miserable and dreams of someone taking her away and loving her. Eponine doesn't do much, but her mother does compliment the hat she wears, to emphasize how Eponine is spoiled and favored, compared to Cosette.

Video Games

 * The first Golden Sun started with Isaac and Garet as kids, during the tragedy in which Isaac's father, Jenna's parents and Jenna's brother lost their lives.
 * Tsukihime's intro is about when Shiki met Sensei right after his accident at age nine, an immensely important time in life for a bunch of reasons. The non spoiler one is that this is when his eyes activated and got his Mystic Eye killer glasses. And learned slicing stuff in half for shits and giggles wasn't nice.
 * Fallout 3's tutorial starts with the player character's birth and there's a short section where one must play as a baby.
 * One of the greatest examples of a Justified Tutorial in all of gaming. The game teaches you how to walk because you are a baby, and as you grow from a newborn to an infant to a preteen to an older teen to a young adult, you learn to interact with things, talk to other people, use items, and (yes) use weapons.
 * Dragon Quest V opens with the main character's birth, and starts you off playing as a five-year-old.
 * Both Fable games set up the plot this way. The first with your town being destroyed and your father getting killed, the second when your sister is killed in front of you.
 * In Breath of Fire II, the player starts out as a child, who goes to find his sister Yua and protects her from monsters, only to return and find out that no one in the town remembers him . He meets his fellow orphan, Bow, and the two become Rangers together years later.
 * Assassin's Creed II has Ezio's introduction as a new-born where we get to move our arms and legs a bit.
 * The Godfather begins with Aldo witnessing his father's murder and the destruction of their family bakery by the Barzinis and Don Corleone's promise that he'll be able to take his revenge. Flash forward nine years...

Webcomics

 * Fetch Quest Saga of the Twelve Artifacts first introduces us to young Saffron Lachesis, when she gets the gift of a cane sword from her father, the King of Strife.
 * The Order of the Stick book Start of Darkness begins with a single page showing us a four-year-old Xykon crying over the death of his pet dog.

Western Animation

 * The Legend of Korra introduces Korra as a pudgy, spirited four-year-old Child Prodigy who has already begun to bend water, earth, and fire, proving beyond a doubt her Avatar status to the Order of the White Lotus. "I'm the Avatar, you gotta deal with it!" A scene of her lighting a small, candleflame-sized fire Age Cuts to her at seventeen in a training duel against two firebenders, clearly having mastered firebending.
 * In Veggie Tales, this is done as a tribute to Indiana Jones and The Last Crusade. "Minnesota Cuke and the Search for Noah's Umbrella" starts off with Minnesota, Marten, and the Scallions as kids.