I have family sharing but my son is being charged for Amazon music when I pay for it. Why is that?

if you're only seeing this on a card statement, check you haven't subscribed to the same service twice, once through Apple and a second time directly with the service provider. E.g., Join Apple Music on your iPhone, iPad, iPod touch, Mac, or PC - Join Apple Music on your iPhone, iPad, iPod touch, Mac, or PC - Apple Support - "Make sure to join Apple Music through either Apple or your mobile phone service provider, so that you're not charged by both."

Click on this link to read what to do "If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement" --> If you see 'apple.com/bill' on your billing statement - Apple Support Note, if you are a Family Sharing organizer, purchases or subscriptions made by another family member may be charged to your card but will not appear under your purchase history or subscriptions. Ask family members about those or check the email receipts Apple sends you for any purchase charged to your Apple Account.

If you need further assistance contact Apple; start here --> Billing & Subscriptions --> https://getsupport.apple.com/?PGF=PGF63006

Do you have a family level plan?