Some good sites for kids of all ages:
Yahooligans
Bonus
Sports Illustrated for Kids
The Locker Room: Sports for Kids
The Happy Puppy Page
Ace Kids
KidsCom
The Last Day of Calvin and Hobbes
Nintendo's Page
Nickelodeon
National Geographic for Kids
SeaWorld
Nine Planets: learn about the solar system
Mr. Potato Head
The Froggy Page
The Crayola Crayon Page
700+ Great Sites for Kids and the Adults that care for them
Kids World 2000: a lot of other links for kids