Minecraft multiplayer

Both of my kids play Minecraft on their cell phones. They used to be able to join each other's world when at home connected to the same WiFi. For some reason, my daughter can no longer join her brother's world, but he can still join hers. My daughter
has a child Microsoft account linked to me and I've gone in multiple times and adjusted the privacy settings but nothing seems to work. This started after I let my daughter buy some coins and she used them to buy 2 new worlds - I don't know if that is the
problem, and if it is, how to fix it. Any suggestions? (I put this under Xbox because I didn't see another relevant category, but my daughter does not have Xbox live - she just plays on her iPhone with her Microsoft login.)

:)