I remember reading an article long ago that originally Kingdom Hearts WAS aimed at children (it's disney, what do you expect).., until hironobu sakaguchi spoke to Nomura and said something like if he make this game aimed toward children the game will undoubtedly fail..
Taking that into account Nomura later changed it and aimed it more toward "older children", probably japanese teen, and thus you got the 2002 Kingdom Hearts
still thinking that it's not "mature enough", Nomura later put a "time skip" in the game to make Sora older.. and you got Kingdom Hearts II
long story short.. no it's not a children's game, but children can also enjoyed it too, it just that children is definitely not their target demographic..
"Well, It's the Black "Waltz," right? You think there'd be three, right?", ~Zidane Tribal, FF IX~