Linux

The only advice I have is to try to make it interesting for them and not just additional practical information they have to memorize. You don't want to be the weird dad that insists on using stuff nobody else does, you have to show them what's cool about it, and also accept maybe they'll just stick with Windows for now.

I also think the main takeaway they should have out of it is that there's many ways of doing the same thing and none is "the correct and only way". They should learn to think critically, navigate unfamiliar user interfaces, learn some more general concepts and connect the dots on how things work, and that computers are logical machines, they don't just do random things because they're weird. Teach them the value of being able to dig into how it works even if it doesn't necessarily benefit them immediately.

Maybe set up a computer or VM with all sorts of WMs and DEs with the express permission to wreck it if they want, or a VM they can set up (even better if they learn they can make their own VMs as well!). Probably have some games on there as well. Maybe tour some old operating systems for the historical context of how we got where we are today. Show them how you can make the computers do things via a terminal and it does the same thing as in the GUI. Show different GUIs, different file managers, different text/document editors, maybe different DE's, maybe even tiling vs floating. What is a file, how are ways you can organize them, how you can move them around, how some programs can open other program's files.

Teach them the computer works for them not the other way around. They can make the computer do literally anything they want if they wish so. But it's okay to use other people's stuff too.