From other replies on here, it seems that you are more interested in the Software side than the hardware. Scratch was the gateway drug for my kids. It immediately let them create games they could share with friends, it immediately let them grab other resources and tweak and change those for their own game.
I also kept them on linux (ubuntu) machines until they started high School - the reasons for this were 2-fold:
1) let them get comfortable with other OS's - they saw windows machines at school and ipads, this showed them that they were all 'just computers' and feel comfortable jumping between os's
2) security - oh man if you could see the amount of dodgy looking .exe and .bat files that are in the download folders.
My kids have now moved on to Godot and pygame, but an old laptop with Scratch was the key.
I also kept them on linux (ubuntu) machines until they started high School - the reasons for this were 2-fold:
1) let them get comfortable with other OS's - they saw windows machines at school and ipads, this showed them that they were all 'just computers' and feel comfortable jumping between os's
2) security - oh man if you could see the amount of dodgy looking .exe and .bat files that are in the download folders.
My kids have now moved on to Godot and pygame, but an old laptop with Scratch was the key.