Hello landon1431,
To add to the answer above — once the family organizer is ready to make the change, here is exactly where to go and what to adjust. The setting can be accessed through the standard Microsoft account page:
- Sign in at https://account.microsoft.com/family with the family organizer's Microsoft account.
- Select the child's account tile.
- Go to Privacy & Online Safety > look for "You can join multiplayer games" and set it to Allow.
- Click Submit, then have the child sign out and back in on their device.
Alternatively, this can also be done through https://account.xbox.com/settings or the Microsoft Family Safety app.