Roblox seems to be the new Minecraft, and since modding is a first party thing, I would guess that’s the easiest place for someone to start.
Minecraft is a bit different because there are two versions of the game. Java, and Bedrock. Bedrock does have some first party coding stuff a bit like Roblox which does seem more aimed at kids: https://education.minecraft.net/ - I think it’s Python based?
Whereas, Java edition is pretty much all third party when it comes to mods and plugins. They are created in java instead. There’s a lot of content out there for it, but not in a way that Roblox or Bedrock has it documented. It’s mainly in the form of YouTube tutorials. There are also some visual editors here like MCCreator which are likely easier for someone young