Here are some fun websites for kids, and reviews of them:
1.Runescape.com - Free online massivley multiplayer fantasy game- 3D
In runescape, you are a human back in the times when there were magicians and people using swords as weapons. You get to fight monsters to raise your level and this game has thousands of hours of gameplay.

neopets.com - Free online magical pet game - 2D

You get to have your own virtual pets, and  get to play games, feed them, and earn neopoints.
miniclip.com - Extremely large collection of arcade games- 2D and 3D

Play over 200 games for ages ranging from 9-12.
clubpenguin.com - Online rpg

PLay as a penguin. Play with your friends, family, or anyone in the world!