how can I manage subscriptions purchased by other family members
We have family sharing enabled. If other family members purchase subscriptions, how can it be managed by the owner of the credit card.
iPhone 5, iOS 10
Those have to be managed through their accounts. In terms of management your only ability would be to remove them as a family member if it should come down to that. If the issue is payment, encourage them to use a gift card to add personal balance to their account. That will be charged first before your card will.