I subscribed to Minecraft Realms and created a Realms server. this required that I create and sign into a Microsoft XBox live account on my phone.
Logging into created realm works fine for me. I invite friends to join the realms server and discover that some friends cannot join the server after accepting the invite.
In troubleshooting I discovered that only people who have an XBox live "child" account cannot log into the realm. People with an XBox "adult" account can log into the realm with no errors. Furthermore, if a "child" account attempts to log into the realm it will disconnect all currently logged in players and make the realm unavailable for login for up to 10 minutes.
I further tested this by converting an XBox "child" account into an "adult" account by changing the account birth date and found that they were then able to log into the realm.
This was tested on iOS, android, and Windows 10 and all versions have the same problem.