I taught a bunch of local kids how to build computer - bought the parts from dabs.com and a cool lit-up case, gave them basic instructions and then stood around saying (truthfully enough) "Feck, I have no idea!" while they worked it out for themselves.
It's really the best way for a kid to get to know about computers, and while not particularly cheaper, it's possible to build up a really good computer gradually by starting with a good motherboard and CPU and improving the parts from there.
Also provides excellent cool points among their friends.