While the predominantly teenage default characters of Chuubo's Marvelous Wish-Granting Engine qualify more often than not (Chuubo, for example, is a geeky kid with a thing for ice cream who occasionally turns into a giant monster and, in the later stages of his arc, gets actually pretty good at that), the real standouts are the rats of Fortitude, whose entire culture is founded on the spiritual ideal of Reepicheep from The Chronicles of Narnia, and who, without ever not being wide-eyed and friendly rodents, still ultimately consider doing cosmic raid dungeons against Eldritch Abominations to be an important cultural trait.
Rook Catchfly in Nobilis is a spunky kid who's going to be a spunky kid indefinitely due to being "stuck" at around nine years of age...and has Aspect 4, which is about enough to beat up the entire US Marine Corps for their lunch money.
Yu-Gi-Oh! has a number of cute monsters that are quite dangerous in the right deck.
The Madolche cycle consists of candy-and-dessert themedLiving Toys with the common ability of getting shuffled back into the deck if they're destroyed in battle or by an opponent's card effect. This means they just keep coming, especially if you have the Madolche Chateau spell card in play, which lets your destroyed Madolche monsters go back to your hand instead of into your deck.
The Guardians of Strike Legion are a species of cat-like Gens who were intentionally engineered by their creators to be adorable. Being Gens, they're targets for the human-supremacistImperium. Or at least they would be, but every single Imperial invasion fleet sent to wipe out the Guardians... hasn't come back. Current Imperial policy on the Guardians is that exterminating them is a lost cause.