You have a born game designer on your hands. It is important to not assume this means they are super interested in programming. They may be, but game design is its own thing. HN will skew you towards programming first, naturally.
Check these out:
- Adventure Game Studio (https://www.adventuregamestudio.co.uk/)
- Twine (https://twinery.org/)
And these have more visual ways of programming which could let them express their ideas with less friction
- Dreams (Playstation)
- Unity (with Playmaker or Bolt visual scripting)
- Godot
And the other suggestions of Scratch are good, but I find Scratch to feel like a way to learn programming more than expressing game design.
Lastly, explore card and tabletop games with them. It’s a whole thing!