CrazyMonkeyGames is the big thing among the 4th graders at my son's school. But I am not convinced. Some of the games appear harmless, but there are some pretty graphically violent games and there is no way to tell which is which. The games aren't grouped according to age or rating and there are so many of them, it's hard to know what your child is doing.
I would rather my child play his Wii or use computer software than freely cruise this website. There's time for that when he's older.