I bought some old refurbished laptops from Dell that my kids play with. Their cost was so low I just let them try their best with a screw driver in the back to open it up. We upgraded ram and relaxed the batty. I threw Linux Mint on there so I have full control without MS advertising to my kids and put retroarch to play some old games. Tons of old games were pretty good for learning depending on your viewpoint. I remember playing Putt Putt in the 90s.
I'm terms of hackabilty, I mean, it will depend on the kids. I try to explain to my kids what is going on inside the laptop and show them it is not magical, but their attention is fleeting. I let my two year play with the screw driver and my 6 year old actually pull ram and replace battery. He likes the old games and has stretched his reading/planning skills to get through Pokemon Red.
Anyways, here is where I frequently find cheap machines.
https://www.dellrefurbished.com/