How to connect a child's MacBook to Family Sharing?

Devices are supposed to automatically connect to family sharing once the account holder is signed in. The account holder is signed in but the device doesn't register on the profile under family sharing, so the restrictions set up don't apply to that device. Under the account on the device, when i click on anything to do with the account, it just shows the spinning wheel. How do I add this device to the family sharing profile?

[Re-Titled by Moderator]