I downloaded this program for my 8 yr old son two days ago. Tried to use it myself and found it to be a little difficult to figure out, but he is already way ahead of me. It turns out he had been watching a tutorial for it over the last couple of weeks. He's already making pistons and machines.
This vid shows what you can do with it. http://www.youtube.com/watch?v=0H5g9VS0ENM