When I gave my kids (9 and 7 year old girls, 4 year old boy) the old computer that I installed with Lubuntu (before it died, hence this thread).... they basically jumped into Chromium and used it to browse to their school's Google Docs/Classroom site. They had no idea what the difference was between Ubuntu, Windows, etc... it all seems to work the same. I think at that age we forget that the interfaces come naturally and for their basic usage any system is more than good enough as long as it has Chrome.
The first thing my 9 and 7 year old girls did was figure out how to change their desktop backgrounds and that was enough entertainment for them. Each had a separate login and user/password which they kept secret, which was their very own, which was exciting for them.... and when they logged in they would return to their desktop background they set up. For several weeks all they were doing was looking for new desktops in Google to change their backgrounds. Amazing what excites kids these days... they ended up fascinated with Poop emojis.
They tried visiting some Flash-game sites to play games and guess what, it didn't work.
(I later installed Adobe Flash for Ubuntu but that's not the point). I was happy that I didn't need to worry about them playing games, they have enough crap to distract them on their tablets, TV and such. I don't want them playing games, I want them creating games... which is why we started working on Scratch and they managed to make some rudimentary stuff with my help, in Ubuntu, using Scratch.
So as far as computers go, if that $74 Thinkcentre M58p 6137 will do the job (Core2Duo 3.0GHz, 4G, 160G, DVD, Windows 10 Home) maybe I should just stick with that for now, put a second Ubuntu partition with dual-boot option and away she goes? I'll plug back in the WiFi USB dongle they were using before, a VGA monitor (I have a DVI-VGA adapter) and the USB mouse/keyboard and they will never know the difference. I may even install Ubuntu Studio this time, instead of Lubuntu, since it is a more powerful machine.