Family Plan for Apple Music

So to start with, I would setup family sharing. https://support.apple.com/en-us/HT201088

Then you need to confirm that you are subscribed to Apple Music Family

https://support.apple.com/en-us/HT204939

Then you need to go to settings > name > family sharing > then look for apple subscriptions and make sure the apple music is being shared.

This resource will help you with sharing your subscription > https://support.apple.com/guide/iphone/share-subscriptions-iph6e7917d3f/ios

hope this helps ;)