Hmm maybe look at one of those kits where you build a computer out of a rasberry PI.

There isn't really anything comparable - there's too much cruft and too many platforms, devices, and peripherals to support, so the simple close-to-the-hardware OS isn't all that practical anymore. Maybe one of the realtime operating systems for microcontrollers are simple enough? Parallax has some products that are fun to play around with as well as being technically interesting and quite capable (they're discontinuing the P2 eval board in favor of their modules, so you can get a decent deal right now, too).

Not sure about your actual goals, but maybe start with something like Pico-8 or TIC-80 fantasy computer? Our 8yo son got an instant grasp of the command line this way; I think at least TIC-80 had a handful of standard Unix commands available as well (e.g "ls").

Next thing I noticed, our son was checking wifi availability on his own via 1) opening the command prompt in Linux Mint and 2) typing "ping google.com". Made my day when I saw that.

I suppose the relevant part is to move step-by-step, and to focus on teaching "Unix mindset" instead of specific tools. TIC-80 or PICO-8 are not bad at all for that.

Good luck!