This isn't a hypothetical question. I'm in charge of buying the christmas gifts for the nephews, and I really want to drive them to someday build a battle robot or a rube-goldberg device or something else really cool someday. They're preschoolers right now, one is 4 and the other going on 2. It's going to be a decade before I buy them power tools for christmas, of course, but now is the perfect time to start laying the groundwork for my evil plan. What would be some ideas to pursue?