I am thinking you may be doing this in a class with Kids? And I am guessing that you are willing to put at least a little time into it.
How about a small battery etc. then a series of switches that you can add in various size rated light bulbs one at a time
If you had say 5 or 10 individually switched bulbs, of assorted sizes, then you can play a game with the kids of which can work, and they will see the various results, kind of cause and effect, and it will draw them into the demo instead of letting them just observe.
I would do it with 12V stuff as that is plentifull, and you could do something like say, a driving light, a tail light (like an 1157 with 2 filaments) a headlight maybe a dome light or clearance light, or maybe some LED trailer lights.
I guess it is all in how much time and $ you would be willing to put into it. I know several stores have helped considerably, or just flat given me stuff when I have done it for my kids school, and explained to the manager what I was doing. Heck, make it nice and the auto parts store may want it as a demo for the guy that comes in and just does not get it why he cannot run his driving lights, car stereo with Mega amp and 8 cell phone chargers of that 10a fuse tap
