We bought a computer for Ryan -- a Raspberry Pi 3. It sounded like it was slow and fidgety enough to potentially encourage some tinkering, but fast enough to actually be able to run desktop software, and cheap enough that if he destroys it, we can chalk it up to a learning experience without much stress. Our goal is to have him put it together from start to finish -- we were inspired by hearing about a family that bought broken down cars for their kids with the premise that if the kid fixed the car, it was theirs to keep and use. In this way, their kids learned to understand how their car worked.
See also: Kano OS