My 10 year old is addicted to Yoobot. It is a wonderful game made by the British Heart Foundation. It allows kids to build their own virtual character and then feed it, give it exercise etc and watch him or her grow up. The message is clear - eat well and do some exercise and you'll stay healthy.
The great thing is, my son is playing the game becasue it is fun, not because it is educational. His friend at school introduced him to it, so I guess that is true viral marketing. An excellent interactive experience for kids. I even noticed that his character is now wearing a BHF branded T shirt that he bought with virtual money online