"The other thing that I did was to put together a desktop from parts with him over the holidays, which was a bit more fun than our usual yearly giant lego build, and I hope will give him an idea that these machines aren't magic-- they are put together, even if I don't have the skills to convey that at the circuit level."

How much did it cost to build? I would wager it was more than $200. To put together a PC is beyond a lot of parents skill.