Thanks for the input, everyone! Last week was a bit hectic with us being at SIGGRAPH, LA, and I'm sorry for the slow reply.
@AmLachDesigns: I haven't seen any signs of political correctness in my kids
Actually the first ideas I heard were pretty much along those lines!
@stonysmith: that's a great idea, that would definitely introduce them to all the basic operations.
@MrNib: how old were these? I'm trying to get a sense of what a good age is to get them started.. For how long could you hold their attention?
@Bathsheba: okay, 'Lego-compatible bricks' then. That's absolutely legal, now that the patent is expired.
@UniverseBecoming: Algodoo looks super cool, but it's a bit too much like a game - they already do enough of that
I already introduced them to Tinkercad and dragging objects around seems doable for them. Reorienting the workplane - not so much yet.. Oh, and I'll share their results, absolutely
Cheers,
Bart