I would also consider use of Gentoo as your Linux distribution to force learning about building packages and command line. I would avoid Arch as that might be too much of a challenge.
For me the goal would be forcing them to learn how this stuff works. I would emphasize scripting in the shell like bash scripts, JavaScript via node, Python, and possibly even Perl. This will take a lot of guidance to get this started because they will need some real world use cases about why they immediately benefit.
Once you get the OS finally set up create an ISO of it and put it on both a thumb drive and home file server. Give the kiddo root access to their own computer and let them really break stuff because you can restore from backup
Edit:
Immediately downvoted. This comment apparently caused a nerd god to shed a tear.