In both Vampire: The Masquerade and Vampire: The Requiem, vampires don't age after being turned. So, in theory, if someone was Embraced four hundred years ago as an eleven-year-old girl, she could have all the awesome might of the most powerful vampire elders and look quite harmless. Similarly, it's noted in a few supplements that due to nutrition, most people were much shorter than modern average a long time ago, so that "short" vampires accorded respect by vampires you believe to be stronger than you should be afforded every courtesy...
In the Pathfinder RPG, there is a feat called Childlike that allows Halflings to appear to be human children, without impairing their ability to be ass-kicking adventurers. On the other hand it also has massive Squick potential...
Halflings and Gnomes might lack the above-mentioned feat in Dungeons & Dragons, but that still doesn't prevent their womenfolk from being cute or capable of smashing people in the face. Especially in 4th edition, where they lack the traditional Strength penalties that prevent them from being good at melee classes — halflings, with their Dexterity bonus, actually make pretty good Bare Fisted Monks.
Given the free form nature of character creation, it's easy to build such a character in Mutants & Masterminds. Canon examples include teen superheroes Nereid, a rare male version in Whoop-Ant, but most especially Changeling, who's a Voluntary Shapeshifter that prefers the form of a teenage Genki Girl.
On a meta-level, very few of these games prevent you in any way from making this type of character. As long as the engine does not link appearance and combat ability in a specific way, someone is making this character.
Discouraged in Call of Cthulhu in editions where Size as well as Strength influences how much damage you do, since bigger hits harder. Discouraged also in that fighting is a desperate last resort in that game, and if you're down to hoping you can punch out the bad guys, things have already gone to Hell in a handbasket.
Discouraged in perpetual Snark Bait target, the terribly sexist, racist, and unplayable game FATAL. Discouraged because females make poor fighters, attractiveness makes you busty and weak (if female), and because you should not play Fatal. Ever.
One of the sample Powers in Nobilis is Rook Catchfly, the Power of Eternity, who is stuck forever at ten years old. She's a cheerful type who loves showing people how beautiful the world is. She's also Aspect 4 in a system where peak human physical and mental capability is Aspect 2, and her Tarot card in third edition specifies that it's probably a good idea to be polite to all little girls you meet until you're certain they can't bludgeon the Marine Corps into unconsciousness with a bear while outsmarting Stephen Hawking.