Maybe an old Amiga. It's easy to get started, as most games boot automatically from the floppy disk. When they are ready for more advanced usage, the default boot ends up in Workbench where they can get a desktop environment.

After a while, expose them to programming through AMOS or Blitz Basic.

Just a start.

https://www.ebay.com/sch/i.html?_nkw=dell+pentium+ii+mid+tow...

doesnt HAVE to be old.

just a regular PC with keyboard mouse monitor and the unit.

edit: maybe a DVD unit for reading AND writing disks.

In this case I gave them a COMPAQ Portable III I rescued from my father-in-law's computer repair shop.

myroom.isfine.org on a pinch. I tried to make everything as authentically annoying as it was, having to rewind tapes shapes character

Look up the Pocket 386.