Ive done something similar before. Went with a videogame and showed them how it editing the code changed the game. They started requesting silly changes and went from there. Super fun.
This is the main reason RMS went from being a developer to an activist. He was (and is) trying to prevent this from happening.
Javascript Pacman.
I have a first grader who now knows how to edit the javascript file in Atom, increase the number of lives parameter, make edits to the map etc...
We also swapped out the pac man with my face, which is a source of continued amusement, especially the death animation.