i love 4 bit cpus, it is so nice to teach with them
i made a card "game" for Richard Buckland's made up instruction set and my daughter was very receptive, we also started to build Ben Eater's cpu, but it was too slow and she got impatient
i think nibbler is the right amount of fun/effort, i will absolutely try it
link to the game if someone is interested (you can also print it yourself):
https://punkx.org/4917