Just got a 200 dollar computer for my 4 yo daughter and I'm putting ubuntu server on it. I'm recreating a modern version of the experience I had with my first family computer, a Tandy 1000 where you would first boot into dos and then use the command line to open programs.
I installed ubuntu server LTS, Xorg server, and Brave browser. Only hangup now is getting the audio working. I installed pulseaudio, it sees the audio device, but when I go to play sound I hear nothing with no error messages.
Her only usecase at the moment is when she wants to watch youtube videos, I'll also put on a drawing program (GIMP or Krita or Tuxpaint probably) and she can use my wacom tablet. She (only) watches educational videos and it really improves her english so I'm happy to do it. Plus I would rather get her on a computer experience like I had growing up consisting of a keyboard and a terminal, instead of the detail-hiding non-technical experience so many kids have using ipads or similar nowadays.