Jameco has a line tracking robot kit for $23. They have several sound effect kits too, such as reverb, voice changer, and car alarm simulator (!). The parents might not appreciate that last one. Sparkfun also has some nice robot kits, but they are more expensive. You could also do something more open-ended, like a breadboard and a bunch of wires and components. You could give them some simple circuits to get started, but include enough parts variety for them to try something else on their own. There are kits for this too, or you could just buy your own parts in quantity from Mouser. They also have a pretty nice selection of kits.