Start with 2D and something like gamemaker, stealing sprites from games like metal slug. If they aren't familiar - setup a mame or snes emulator and go to town.

The key is to reduce the barrier to entry and get something emergent to happen. Like a zombie shooter where a shot zombie stumbles into a fire barrel that adds "fire" to anything it touches. This could create a flaming zombie that spreads fire to the other zombies. Boom, the kid is hooked and before you know it you're paying for expensive and dubious game design curriculum cursing my existence.