Platinum Arts Sandbox Free 3D Game Maker is a popular award winning open source free game maker and educational game design software tool that makes it easy for kids and adults to create video games, maps and worlds.
I always look for interesting softwares for my kids. You know, something they actually could learn from and got skills to use in other terms off life. They want to play games, ok, why not let them build something for them self then. :-) @safegaard