Most people are posting links to hardware, but in my mind, this is a software question. Maybe one of those BASIC interpreters with an integrated IDE, the way that QBASIC used to be on older Windows versions?
Your kids won't literally need to navigate the command line to launch the games. But I think that editing config files to "hack" the game is still on the table :-)
Responding to my own comment: maybe it does have to be separate hardware. If your kid is programming on the same computer they use for Fortnight, then whenever they lose momentum and get discouraged, they're going to be tempted to switch over and play a nice shiny AAA game instead. Or hop on social media, etc. Modern computers just have too many distractions at hand.
Having a separate computer just for the lo-fi programming environment won't fully prevent that, but at least it's a minor roadblock.
I think it has to be a different HW. Iroamed a lot about it, and have your same conclusion.
To your parent comment, yes. Is about SW. And the problem I see, ist that today we have super conputers with studio level audio, and cinema quality video. But programing anything other than a “hello world” or basic text application, is so difficult and complex, that no average kid will endeavor such a trip. Frameworks, Libraries… full of rabbit-holes that I do not dare to enter being a professional programmer.
We need some simple language that allows to bild interesting things relatively easy… like endbasic in other comments.
QB64 is the modern equivalent: https://qb64.com/
Your kids won't literally need to navigate the command line to launch the games. But I think that editing config files to "hack" the game is still on the table :-)
EDIT: also https://www.endbasic.dev/, which jmmv posted below.