Little Miss Almighty

Since most people imagine God as an bearded old man, the natural antithesis of this expectation would be having the supreme ruler of the universe be a young girl instead. Creators will sometimes do this to subvert a viewer's expectations, especially in works heavy with Epileptic Trees and Mind Screw.

Compare Divine Race Lift, Goo-Goo Godlike, Little Miss Badass.

Examples of Little Miss Almighty include:

Anime and Manga

This is a fan theory about Haruhi Suzumiya. It's mentioned only once in the show as a suggestion, not as truth, and without evidence. This also wouldn't fit the trope because Haruhi is not young in relation to the other characters. However, it's become a persistent meme.
Lain from Serial Experiments Lain.
- She personally denies it quite vigorously. The implications are that she could be God if she wanted to, but doesn't think that's what the world needs. There also may or may not be something above her better deserving of the title, but he isn't exactly a conventional deity either, assuming that he's not just her creation, as well. Mind Screw, ahoy!
The premise of Kamichu!!. The full title actually means "The Goddess, A Middle-School Girl"
Kamichama Karin has a variation, with a bunch of middle-schoolers who can borrow the powers of various Greek gods using magic rings.
Madoka's wish in Puella Magi Madoka Magica essentially makes her God. Well, that and Hope
Hanyuu of Higurashi no Naku Koro ni is the true identity of Oyashiro-sama, and she takes the form of a young girl, appearing no older than Satoko or Rika.

Comic Books

In The Sandman, the Lord of Chaos, Shivering Jemmy, manifests as a young blond girl with a balloon.
- Also Delirium of the Endless, who at first appears to be a crazy yet harmless young woman, but you really, really don't want to cross her.
The ultimate conclusion of Lucifer: Yahweh retires by Lucifer's suggestion, and Elaine Belloc takes His place.

Film

Cafe was a movie about a coffee shop that took place in a world as a computer program. The chief programmer was a teenage girl.
Alanis Morissette is God in Dogma, though she is more like a young adult woman. This is partly Mind Screw, and partly Rule of Funny.
- ...but she is so adorable! blerp!

Literature

Aphrael, the Child Goddess in The Elenium.
The Ellimist, who "merely appears all-powerful from your limited perspective", once appears to the group by copying the form of a nearby girl.
Ivy (The Archive) in the Dresden Files contains the entire wealth of any knowledge that has ever been written down in any language throughout time... in the body of an 8-year old girl... who likes kitties.
Not exactly a god, but The Neverending Story has Moonchild.
In Percy Jackson and The Olympians, Artemis and Hestia sometimes appear as pre-teen girls

Live-Action TV

On Angel, Wolfram & Hart's conduit to the Senior Partners would sometimes manifest as a sweet little girl
God took the shape of an elementary-school girl a couple of times in Joan of Arcadia.

Music

The song, God Is a Girl.

Tabletop Games

It is implied in Demon: The Fallen fluff that the (old) WorldOfDarkness God is a girl. Since the Fallen are pretty much the only still-living beings who have actually seen God at the dawn of time, their word can be trusted.
- Since the vast majority of the Fallen are still really, really pissed at God, don't put too much stock in their word.
- God also has an evil sister named Oblivion, short for "The Grand Maw of Oblivion." Grand Maw wanted to create her own universe, and saying that she "failed" is not putting it strongly enough. Now she and her "offspring" spend their time gnawing underneath the afterlife until the end of the world.

Video Games

Eternal Daughter: Player character Mia is a demigod.
Not technically a god, but the Fairy Queen in The Legend of Zelda the Wind Waker manifests to Link in a child-like form. And then hits on him.
- Zelda, the reincarnation of the Goddess Hylia from The Legend of Zelda Skyward Sword definitely counts.
Moriya Suwako from Touhou.
- And hey, Kanako Yasaka, why not, even though she's not as young-looking. Reality Warper Yukari Yakumo also may qualify. She's not God. But she might as well be.
Yune from Fire Emblem: Radiant Dawn should count.
Taken to the extreme in Ar tonelico series. Humans (and more) live in three gigantic towers that are also massive computers capable of potent magic [sic]. The Administrators of the towers are all little girls, especially Frelia.
- Interestingly, the Satan (?) equivalent of Ar tonelico 1 Myuru/Jakuri is also a little girl.
Oracle of Tao has Ambrosia, who for much of the first game is oblivious that she is God's avatar (she's about 20-30 though). This creates an odd situation, since she was literally sending herself on a quest. Later on though, she accepts being God fully, and the current version of God gets reincarnated so that Ambrosia can have a life with Nevras while her mother can have a life with God.
A sort-of inversion: In Strange Journey, Lucifer appears as a little blonde girl in white dress, calling herself "Louisa Ferre."

Web Comics

Minus isn't really confirmed to be God in the comic - just a girl with limitless unexplained powers.
- But she does create the Afterlife, and is shown to run it whenever she isn't busy having fun.
An old, discontinued webcomic, Acid Reflux, had a little deity girl get her own (used) universe as a present. Her name, of course, was "God". Much humor occurs when she's sucked into said universe, a sort of high fantasy pastiche.

Web Original

In BitF City, the most powerful Randomnian god, creater of a whole dimension, is a 6-year old. She also has the mentality of one.
SCP-239 from the SCP Foundation seems to be a Reality Warper in the form of a young girl.