Your Apple TV+ subscription is linked to your Apple ID account, and that should link the Apple TV+ subscription to all members of your Family Sharing group by their Apple ID account, as specified during setup of the Family Sharing group.
Verify that each group member signs in to their device and/or the Apple TV app using their own Apple ID account, as specified during setup of the Family Sharing group. Not being signed in for the Apple TV app, might cause content to be unavailable. Don’t share the account itself, as the point of Family Sharing is to have these perks on each their own account.
Then it should just work.
Family Sharing - Apple Support
Set up Family Sharing - Apple Support
If you still experience issues after reviewing the above, then contact Apple Support.
Contact Apple for support and service - Apple Support (phone numbers)
or Get Support - Apple Support (start your support request online)