Hello tiphaine PROST,
Welcome to Microsoft Q&A forum. I'm happy to help.
As I understand, you're trying to enable playing game online to your child account. That option lies in the Privacy settings of the child account. To access and change it, please follow these steps:
- Go to xbox.com/privacy and log in with the parent account.
- After logging in, please find your son's gamertag in the list and select it.
- Change to "Xbox and Windows 10 online and privacy settings"
- Set "... can join multiplayer games" to allow.
- Scroll down and click "Submit" to save the changes.
- Please note that the changes might take a while to be applied, so please try to log out and log in again if that doesn't work right away.
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