Protected by a Child

"If I have children and subsequently grandchildren, I will keep my three-year-old granddaughter near me at all times. When the hero enters to kill me, I will ask him to first explain to her why it is necessary to kill her beloved grandpa. When the hero launches into an explanation of morality way over her head, that will be her cue to pull the lever and send him into the pit of crocodiles. After all, small children like crocodiles almost as much as Evil Overlords and it's important to spend quality time with the grandkids."

—Evil Overlord List, #143

"Using a little girl for a shield... This is low even for you!"

—Dr. Jumba Jookiba, Lilo and Stitch

So The Hero is on his Roaring Rampage of Revenge, and find the man who killed his Mentor, his Loved One, his Small Annoying Creature... or is just an evil bastard.

He got the Villain on the ropes, and goes to deliver the finishing blow, ready to stop the evil once and for all... what's this?!

Where did that kid come from?! The child of course doesn't want anyone to hurt their family, and refuses to move.

Usually the Hero backs down - with some amount of hesitation depending on the guy's place on the sliding scale of morality, after all - the kid doesn't need to be traumatized. This invokes a realization of what he's doing. Sometimes ends in An Aesop about Revenge.

Oddly enough, due to Harmful to Minors trope, you rarely see a full subversion except in the grimmest Black and Grey Morality worlds.

A nasty variant has the villain invoking this trope by using a child as a Human Shield or hostage.

Contrast Daddy's Little Villain. See also Infant Immortality.

Examples of Protected by a Child include:

Anime

In Pokémon 3: Spell of the Unown, a little girl protects Entei and screams "No More Fighting!". Granted Entei isn't evil - just a bit too much of a Literal Genie.
- In the regular series itself, Brock's siblings pull this on Ash to get him to stop attacking Brock's Onix.
Fullmetal Alchemist: Ed and Al discover, to their horror, that five-year-old Nina Tucker's father has performed a transmutation on her and her pet dog to make them into a chimera. Nina's father Shou tells Ed that the two of them are Not So Different, however, as Ed tested the boundaries of alchemy as well by trying to revive his mother. Ed screams "I'm not-like-YOU!!", and starts to beat Nina's father raw in a rage - until the chimera Nina starts to growl and tug at his sleeve; "No, big bwudder." Even Al couldn't stop him before this.
- In the manga, Scar of all people gets protected by a girl from Xing named May Chang.
As part of a Q&A session in the bonus features of the Weiss Kreuz DVDs, voice actor Tomokazu Seki half-jokingly suggests that this is probably the best way to beat his character, Ken, who is a Friend to All Children.
Black Star's first mission in Soul Eater ends with him defeating the bodyguard Mifune, only to be 'attacked' by his charge, very young witch Angela, who insists that he leave Mifune alone. Shocked that the witch he was supposedly sent to kill is a little girl, the would-be assassin backs down.

Comics

In Watchmen, when Rorschach confronts his ex-landlady about her having lied about him. Although none of them directly jump in his way, the presence of her children makes him back down.

Film

The Bride from Kill Bill had a thing about not killing people in front of their children. Her opponent Vernita Green orders her daughter Nikki upstairs. When Vernita makes her attempt to bushwhack the Bride, she puts a knife into her at range—and turns to see that Nikki has seen the whole thing.

The Bride: It was not my intention to do this in front of you. For that, I'm sorry. But take my word for it; your mother had it coming. When you grow up, if you still feel raw about it, I'll be waiting.

In Terminator 2, Sarah Connor is about to kill Dyson to prevent him from developing Skynet, but his son throws himself in the way, screaming, "Don't hurt my daddy!" over and over again. In that moment, Sarah realizes what she was about to do and has a bit of a breakdown, giving her son and the T-800 time to catch up to her.
In the original Stargate movie, when presented before Ra, Jack manages to wrestle a weapon from the guards, turns to shoot Ra... but Ra's entourage of children calmly step in front of him, and he can't bring himself to shoot through them.
- And then he nukes them, but we're supposed to forget their existence by that point plus Ra had established his credibility as a world enslaving, attempted genocide level villain. In the Novelization, the oldest of them gathers the rest and they jump ship before it blows up.
The Fall toys with this. Although Alexandria is on the hero's side, she still essentially plays this role during the final battle.
The villain of The Bourne Identity is revealed to be saved by this trope for a while, allowing him to cause the injury that lead to the titular character's amnesia in the first place.
In The Dead Zone, Smith tries to assassinate a Presidential candidate, but the man uses a child as a shield. While Smith gets taken down by security, the candidate's cowardly actions ruin his chances at election.
In Let the Right One In, Oskar is saved from the bullies at the last moment by Eli. Although Eli is, admittedly, not a normal child. That said, he's already saved Eli from almost certain death, so this film has two examples of this trope protecting each other.
Although she was an adult at the time (twenty years old) the trope is otherwise played straight between Elena and Diego in The Mask of Zorro. When Diego is about to kill Rafael, Elena throws herself between them, begging her biological father not to kill the man who raised her.
At the climax of Rise of the Guardians, it is Jamie and the other children stepping forward to defend the weakened Guardians which sets in motion Pitch's defeat.

Live Action TV

On Lost, when the survivors have Ben captured in season four, Sawyer says that they should just shoot him and be done with it, and Locke responds that he's not so callous that he will kill him while his daughter is right there watching.
- Another occurrence of this is when Ben is about to shoot Penny but is stopped when her young son walks out of the boat, and hesitates just long enough to get soundly thrashed by a very irate Desmond, who he had just shot right in the groceries.
An inverted case of protecting the heroine from the Dean Bitterman in Japanese drama Shokojo Seira. Little Romi is the only reason why school Director Mimura Chieko kept Seira in the school or otherwise she would have been kicked out.

Video Games

Shadow Hearts: Covenant has a rare double-subversion of this trope: Yuri, upon meeting Ishimura, who was planning on adding a new front to the war, and thus more suffering (A rather slimy fellow). As Yuri was about to attack Ishimura, he was confronted by the guy's grandson. Instead of backing down, he pushes the kid aside, and brutally kicked the old man repeatedly, but the subversion gets subverted when he breaks down and is unable to kill him.
In Disgaea, after defeating Hoggmeister, Laharl is about to kill him when the pig's son begs him to stop. If the player's ally-kill count is low, Laharl automatically spares the guy (who joins). But if your kill count is high, you can go to kill him but Flonne will attack you, leading to a Nonstandard Game Over
Nonviolent example in Persona 4; Dojima occasionally presses the protagonist on what he does with his time, with increasing hostility; his daughter Nanako (who also happens to be the Justice Arcana) is an expert on shutting him down.

Web Comics

Done without the child's presence needed in Dominic Deegan. Celesto, having Jumped Off the Slippery Slope, slaughters a Jerkass athlete and turns to kill the stripper he was accosting... only to get a vision of her telling her daughter good night.

Celesto: Go home, hug your daughter tight, and thank her for that good-luck pinch. She just saved your life.

Web Original

Done the other way around in "Ayla and the Birthday Brawl" of the Whateley Universe. When Generator and Bladedancer have to dive into a supervillain hostage crisis and one of the Felonious Four is about to kill Generator, one of the hostage children steps forward to save Generator. It only partially works. The supervillain is going to mutilate the kid then kill Generator, but spends so much time threatening them that he runs out of time and has to use the escape portal before he can hurt either.

Western Animation

In Justice League Unlimited, Huntress decided to kill the man responsible for her parent's death before he could go into hiding, the villain was literally at her mercy when the man's son broke free from his escort. She ended up turning her bow away, sparing his life.
- This is partly because she was reminded of the night when her parents died.
Near the end of the South Park movie, just before Terrance and Phillip are about to be killed, the kids stand in the way and plead to the army and Kyle's mom not to kill them. This doesn't do much good, as the kids are only about a third T&P's height and though Kyle's speech touches the soldiers, Kyle's mom shoots them dead anyway.

Real Life

Some people actually try this in Real Life. Unfortunately, it has a tendency to result in dead children. To everyone reading, remember that while this may generate horrific publicity for the erstwhile child-killer, you're an evil, evil, evil sonofabitch and you shouldn't have done that. Seriously, not cool.
- The Westboro Baptist Church bring their kids to their protests, just for this purpose.
- Some members of the Occupy Wall Street movement tried this in Washington, using kids to barricade the doors of a convention center and block traffic in the street.
Baboons, while trying to avoid a fight will hide among infants, as most Baboons won't fight in front of them.
- It's a social survival tactic to get in good with mothers and provide babysitting service for them. If you're carrying an infant, you're largely immune to attacks from other males. If a male does disregard an infant and attack the babysitter, the females will swarm him in defense of the infant.
Recently, there was a news article about a seven-year-old girl and her mother who were attacked by the mom's ex-boyfriend. When he attempted to kill the mother, THE GIRL TOOK SEVERAL BULLETS! Amazingly she lived, and the boyfriend will probably get life.
I seem to remember a story that one of Saddam Hussein's tactics was to place his command centres on the top floors of schools to discourage attacks on the commanders. (I could be wrong about this, so please correct me if I am.)