Scratch is "games for kids." They are very prescriptive and have guides - you know, lesson plans - for everything they can teach. Either it supports Steam Deck or it doesn't.
If they're 14+ Unity is realistic and is the same thing the pros use.
I haven't tried Unreal Engine for Fortnite but if they play it, it is probably the best option.
Minecraft, Bloons and Terraria modding is also pretty good.
You can give them a GPT4 sub and it seems to do a good job answering basic computing questions if this is going to be their first time e.g. installing a professional tool.