1. Games with highly modifiable level building tools: Game Maker's Garage, Mario Maker, LittleBigPlanet and (a big step up) Dreams 2. actually playing some indie games with them to try and give them an idea of what's remotely feasible 3. if the latter gets them interested in 2D possibilities focusing on pixel art and having you piece the bits together 4. if they're still interested start looking at some game making tools; they might want to just focus on pixel art or music or who knows what at that point instead too?

Really the hardest bit I see is shifting their thinking on what a game is; atm it's probably ramming in every idea they have when it needs to be focusing on one idea and refining the hell out of it.