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.
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)…
Vampire The Masquerade Bloodlines Deus Ex Hotline Miami Unreal Tournament Journey Cyberpunk 2077 The Witcher Jet Set Radio All the Silent Hill (even Downpour) Colony Wars