It's lazy but I set my 2yo up with a thinkpad running ubuntu that boots directly into terminal and has a bunch of single-letter aliases to open a few very specific sites including a sketchy in-browser dos emulation running math blaster.
Would have loved to actually just get a dos machine running for him but time starts moving so fast after they're born...
Anyway he's 5 now and still loves getting into some Math Blaster, and it's still providing plenty of challenge for him, I forgot how far it went academically.
Interestingly I think this giving kids this kind of retro software has the positive side-effect of boring kids after a reasonable amount of time. I've been thinking of it as finite media.
He loves Math Blaster but will often choose a physical math workbook instead when its math time, and he stops playing after an hour or so at most so I can just give him unrestricted access to his thinkpad and he organically self-regulates screen-time. Noticed similar effects giving him my old gameboy pocket and an old 8 inch CRT with a built in DVD player and a stack of random Reading Rainbow DVDs. Also using a ThinkPad or Raspberry Pi that struggles at least a little bit with streaming YouTube has helped keep that from getting out of control since I couldn't ultimately avoid youtube for some no-dvd-available science / math / music video stuff.
I highly recommend Eureka, Dirk Niblick, udiprod and Minds Eye.