This one has been on my radar as a first computer for my son for a while - just lock out the wifi and set it up with a "boot to basic" image maybe?

Certainly something which could grow to support some Arduino work.

EDIT: Admittedly this would be a no brainer if there was an off-the-shelf Atari ST style thing - https://en.wikipedia.org/wiki/Atari_ST purely for the sheer mass providing some protection.

Kids don't want "boot to basic". Old fogey's like me brought up on Commodore 64 look back on boot to basic like the good old days, but "boot to basic" is long gone and no-one is interested in that, except conceptually old people think it makes kids learn because that's what made them learn, but back in the old days you tolerated boot to basic in a world in which the only computer for 3 square miles was the BBC micro sitting in front of you.