I have a Microsoft Family account and have my daughter as one of the family members.
Whenever she tries to play Minecraft (Java Edition) with us, it pops up something like the following:
"Multiplayer is disabled. Please check your Microsoft account settings."
I tried all kinds of things found online. They kept saying to go to the Xbox Privacy and online safety screen, but I couldn't find the "You can play with people outside of Xbox Live" and "You can join multiplayer games" settings. Well, that's because I needed to click on the "Xbox Series ... and Windows 10 devices Online Safety" tab.
Here's a direct link. You will need to replace the "GAMERTAGHERE" with your child's gamer tag. You need to be logged in under your (parental) account, not the child's.