Family Sharing is such a "bolted on" experience. Its obviously been hastily glued together. I really wish it was a more polished product, because I run into annoying issues constantly.
I stick through the hassle cause when Google killed Google Play Music I tried out Apple Music by getting on my parents' family sharing and don't mind having one less subscription to pay for as long as they don't care. But it is annoying it puts my account in a state where I can't buy any subscriptions or in-app purchases without it getting charged to daddy's credit card. The workaround I've found is you can go on the Apple Store app/website and gift yourself an Apple gift card and claim it on your account. Then it'll charge to those funds first before the family organizer's credit card.