Is your brother making games? If yes : there are several easy to use game engine like mentalo.app, Godot, gdevelop and such.
If your brother is mostly playing, then anything by Nintendo would be a good idea. Old consoles are cheap and if you have the Switch there are a lot of really good game on it.
Edit: And I second Minecraft or the free version (minetest I think)...
While I do play Roblox (because we don't have a federated alternative for it yet) I had concerns about it - moderation, playerbase maturity issues (yes, while its being taken over by young adults the majority is still unmatured), updates that the community doesn't want, and "racism" against other platforms e.g. Linux. You're right about how they exploit the creators.
Do fedi or Foss games really need a roblox alternative ?
I'm not sure...
Do roblox teach you how to make games? Well it teaches you do build games for roblox that's it.
Is roblox the only place to find games for kids, surely not.