Thinking about your kids „safety“ (that’s a strong word, but consider gambling, violent content, scammers, etc.), Minecraft is definitely a better choice for a child. Get him into Java Edition rather than Bedrock edition and most of the mods are free, there’s a lot of tinkering he can do with it and practically all of the games content single- and multiplayer is absolutely child-safe.

Roblox is a lot more open-ended with what it lets it’s users create, which leads to a lot of violent and not safe for children content, mixed with some pretty aggressive monetization. I don’t have a child, but if I did I’d rather let them play Call of Duty (one of the older ones without microtransactions) than leave them alone with Roblox.

Mixed bag. Lots of scammers as well. Also lots of tears.