In terms of gameplay and inferface, I don't think Kingdom Hearts is bad that way. You lock onto a target automatically, and you attack with the attack command. Although I admit, movement was a tad awkard at first, but its nothing near as bad as it was in FF10, and you get used to it fast. The only problem inferace wise is the camera, and thats hardly a problem that only KH has to deal with, as its common.
And the story for me at least, picks up in the latter half of the game. To tell you the truth, when I first started playing KH, I got to Atlantica, and got to where you fought Ursala. I basically dropped the game for a few months, not wanting to continue (aladins world killed my enthusaism). Later on, I picked it back up, and soon got back into the game, and beat it. I really liked the second half of the game.
As for "kiddy", most of that ends with the source material, or a few dumb jokes here and there, but (like you said) most games/animes have things like that, even the more serious ones.