I don't want my child to play multiplayer on Minecraft since he joins random worlds that belong to strangers, are there any ways to make it so he can't join servers?
To disable joining servers, you can simply turn off internet on the computer. If you don't want to do this manually or think that the child can enable it on their own, you can blacklist some Minecraft related things in a firewall, basically the opposite of this question. You could also choose some not commonly used, but still stable version, because you can only join with the same version that the server is on. I would for example recommend 1.12.2-pre2 for that, it's basically the same as 1.12.2, but not compatible with 1.12.2 servers.
You can also change chat settings to make the chat completely invisible.
If you want the child to have an easier game, you can lock it into "Peaceful" difficulty.
If you don't want the child to exit the world, join a different one that you play on and destroy things on there (TangoTek used to have a lot of problems with that), then you should make a new launcher profile that uses a different
.minecraft folder, that way the other worlds do not appear in the world list.