Normally if you have just created a new Microsoft/Xbox account for a Child it has default privacy restrictions that won’t allow them to connect to an online server to play Minecraft with their friends.
Here is a quick video how to change those restrictions:
- Login to aka.ms/accountsettings (make sure you login as the parent account you used to create their account)
- Once you are logged in, click on your child’s picture/avatar
- Click on: Xbox One/Windows 10 Online Safety
- You can play with people outside of Xbox Live
- You can join multiplayer games
- Click Submit
You may find that when you go back into the app, it has cached the settings so it may be necessary to sign out and sign back in as your Child.