Yesterday, I let my elder son of almost 7, fool around with 2 less obvious programs. He had hours of fun with both.
The first is Google SketchUp, a drawing program with a simple interface, that lets you draw in 3D. He had hours of fun drawing houses.
IMO, Google SketchUp is a revolutionary program in GUI design. Google solved the problem of how to manipulate 3D objects in a 2D environment very elegantly.
What I like a lot less, is that they went all the way, ignoring direct entering of data, as it seems to be quite impossible to directly enter the dimensions of objects. For example, if I want my door to be exactly 6'8" high, why do I have to drag at it and check its dimensions, and retry until I get it right? Why can't I just enter the desired height? Maybe I just haven't found it.
The other program he had hours of fun with, is Stellarium. He loves to select a planet, and then zoom in until you can see their moons. Just gorgeous.