This is probably one of my favorite Kingdom Hearts games besides the first and second installments, but I think that if you think your kid is ready for the second Kingdom Hearts, they should be able to play this one. If they haven't played Kingdom Hearts at all, this game is the prequel so they can play this and then start the actual series.
The violence is kind of intense for some children though. There is a scene where an old man stabs himself in the chest with a giant key sword, and his heart comes out (this is not at ALL bloody) and he possesses one of the main characters, and you have to fight back for your body in your mind. One other battle scene has a sniper character that shoots purple like darts at you, and after you defeat him the character blasts darkness on him and his face is badly cut and has a couple of scars where he was wounded. The character that did that also killed his master when he tried to kill his best (good) friend because he could cause evil.
There are also positive role models like Ventus and Aqua that are trying to defeat and evil force called the nobodies which are monsters that you can kill (they disappear into darkness) in a third person view.
They are trying to kill this evil with healthy teamwork and love each other and sacrifice for each other.
The only consumerism in the game is the occasional Final Fantasy characters, and the more constant Disney characters that pop up in the game.
There are good sides and bad sides to this game, but the only thing you should really worry about is the violence part.
I hope this helped (BD)