My just turned 6 year old loves spore up to the space age, enjoys all the scooby doo games, all the simple puzzle games such as bejewled. He has finished alot of the movie tie-in games such as ice age 2 and shrek 2 & 3. (all for the PC).
You could check out "Second Hand Lands". It's a fairy tale MMORPG, a parody on fantasy MMORPGs and fairy tales, in some ways similar to Shrek. You play as a wolf, sheep or a two-legged furball (squirrel, skunk, bear, cat or whatever). One of the most unique aspects of the game is that players can ride each other if one player is wolf/sheep and another a furball. You can kill things (piggy banks, princess pwnys and other hilarious stuff), save kittens in distress, help mafia boss Don Humpty Dumpty to defeat Salmonella, milk monkeys, gather resources for crafting, help seven gnomes to hold a decent tea party, raid boss monsters at higher levels with other players and do other stuff.
I am not sure what the age ranges are for this game but check out Crayon Physics. There is a free demo to test it out. I had a blast with the demo. The object is really simple, get the ball to the star but leaves lots of room for imagination.