I have just found a very interesting little site at www.secretbuilders.com

A virtual world for kids full of education games. You create your own character then play the educational games to win “shills” which you then use to buy items for your house, or pets! You can also talk to other children and click on items to learn interesting facts.

This all happens within your web-browser. There is nothing to download or buy, you just need to sign up.