Children and their friends cannot log on to realms

Bought and created a Realm. Invited my kids and their best friend. When they try to join, all receive an error message:

"You cannot play online multiplayer through Xbox Live because of how your account is set up. This can be changed in your privacy & online safety settings on Xbox.com."

I changed my kids' privacy settings and then they were unable to log on to Xbox Live for a few hours. Now they can log on again, but still cannot join the Realm and get the same error as above.

I am on an iPhone SE, kids are all on iPads.

We've used Realms before without issue.