How can I do the necessary incantations to allow my child to play minecraft?
For Xbox and on non-Java edition there are instructions (follow link). They say to allow the child's account to join clubs, and such.
1. Log in to https://account.xbox.com/settings with the parent’s Microsoft account. 2. Click the child account's profile. 3. In the navigation tabs, click Xbox Series X|S, Xbox One, and Windows 10 devices Online Safety 4. Select Allow for You can create and join clubs, and You can join multiplayer games.
I have of course enabled this, but Multiplayer is still greyed out when launching Minecraft with their credentials. It works fine on other accounts, it's definitely that Microsoft/Xbox are blocking the access.
I had to change their account to allow "ESRB 10+" in order to install from the purchased code.
If it helps we don't have an xbox, don't use xbox live or any other Microsoft gaming, and never will based on this experience it is so, so convoluted just to play a game.