+1 for Mint, our 8yo son finds his way around nicely with it. I definitely had a "Maybe I'm actually a pretty cool dad" moment when I noticed him checking wifi availability on his own via 1) clicking the terminal icon and 2) typing "ping google.com" as if he had been doing this stuff for years.
Mint + Pico 8 or TIC-80 for explorative gaming has been a fairly good formula for us. I'm thinking of writing an oldschool text adventure on TIC-80 with him, so he'd 1) learn command line input and 2) develop language and grammar skills at the same time. Computers are not all that bad, yknow.
Prior to the current combo, I was thinking of some FreeDOS based system, because of its simplicity, and since cheesy Intel machines are available for nothing. I also considered this homebrew Z80 computer with BASIC -- for wich we, on all those romantic Father And Son Evenings, would build extra-primitive, ultra-distraction-free text adventures: https://rc2014.co.uk/
But, eventually, I realized that First-Graders These Days are already socializing online, so it is (unfortunately) fairly impossible to avoid something like Discord etc. For that, a machine with only DOS, some Forth or BASIC obviously won't do, but oh well. Now our son is playing a lot of Minecraft, including on servers, but his online behavior seems surprisingly mature or careful. Our household rule still states that he must not spend more than 1 hour on gaming every day, but, obviously, he is fairly creative in finding ways to "show Dad that Minecraft castle, but, uh, hey, I just noticed it needs 15 minutes of improvements, hold on".
Overall, I have found that our son seems to naturally prefer games with low-res graphics. He doesn't like ultra-realistic stuff. Really happy to notice this -- it's kind of like reading a book or drawing where the "realistic movie" is running only in your head. This is great for thinking, I guess.