Content review for this game:
Pertaining to content listed in the upper left corner of this site.
(more detailed re-review coming soon...)
Violence: Take one part Disney animation, and one part Final Fantasy mythology, and you have Kingdom Hearts. The violence never exceeds that of original source material, that is; the world and story of Mulan remains the same, and so on. If you let your kids watch Disney movies, then you will be fine with this game. It's very kid-friendly.
Closing comment: All that said, this game is an RPG (Role playing game), thus is very deep, somewhat complex and long in gameplay. It could take 30-60 hours just to complete the main storyline, let alone the extra you can unlock afterward or during the game. Content-wise I'd recommend this game for ages 7+. But because it is a game even adults can appreciate, thus being more complex than your average kids game, I have to recommend this game for ages 10+.
Just to add one more thing; there is a difficulty setting just for kids, so if you think your younger kid won't get too bored with the game, let 'em play it. So, in this case, 7+.