For the computer thing, that's pretty much exactly why Raspberry Pi was created, so there was a cheap open (relatively) computer that kids could play around with, and even if they somehow manage to break it, it's only like $30.

As for the XBox, couldn't you just supervise while your kid hooked it up? Hand him/her the parts, tell him/her to go to it and you'll answer questions and keep an eye on things so he/she doesn't break anything but otherwise leave him/her to figure it out.