I have in my mind the idea that I start them off from an offline barebones command-line interface, and then we gradually learn file management and maybe other things in that environment while it is still shiny. At some point the child is familiar enough with the computer that we install X or Wayland or whatever it may be. This is mainly in order to start them off in a fairly safe environment until they have shown they are able to handle more.
The questions I have are:
(1) Is this a good idea? I'm aware they'll probably run into Windows as soon as they enter elementary school, but I'm not too concerned they'd fall behind their peers.
(2) Which OS would be a good fit? I had my sights set on NetBSD at first, but then I learned its non-graphical interface does not support Unicode, which is probably a dealbreaker since our native language uses non-ASCII characters.[1]
I'm still leaning toward a BSD because it would be more of a complete, stand-alone operating system than a Linux distribution, but I'm willing to be convinced otherwise.
[1]: Not that I'm planning to localise the OS, but I suspect we'd start off creating files with little messages in our own language.
Play is far more important than conforming to definitions of mimetype for things that are all the same or following someone's arbitary rules for where to put collections of names that mean nothing.
Try NoBSB: Go for walks in the forest or outside, play games, have conversations.
Install Linux Mint, download GCompris. It'll teach your kid how to learn the mouse and keyboard in a minimally stimulating way.
I doubt command line would be most peoples’ first thought, but if that’s your best idea it’s about as good as anything else…this is not a critical decision and there are no technical mistakes in any meaningful sense. Good luck.