I don't really think it'd make it a complete "kiddie" game, but it's certainly a little more childish than Final Fantasy.

I've only played Kingdom Hearts 2, and not once did I feel like it was too childish or babyish for me. It's like an RPG for beginners/younger gamers, but a cute and enjoyable game for older gamers to play. Especially if you're on a break from the more harder and/or indepth RPGs, or are from the early-ish Final Fantasy and Disney era.

Kid is the wrong word, but using any other wouldn't make sense because it's a standalone game.