Family purchase share, but without payment method.

If the family members are all adults like is the case in my family, you do not get a message when they buy something. The way that you know they bought something is that you, as the organizer, get a receipt for the purchase via email. Even if they make the purchase with their own balance from gift cards, you get the receipt.

If you have children under 13 in the family, you can turn on ask to buy, where you have to approve all of their purchases. Take a look at this.

Approve what kids buy with Ask to Buy - Apple Support

If you are the family organizer, you must enter a payment method using a valid credit or debit card in order to set up and invite others to join the family. However, you don’t have to pay for purchases as along as all of the other family member redeem Apple Gift cards and pay for their own purchases that way. We have been using family sharing for three years and my wife and daughter redeem gift cards so my debit card is never charged.

However, as the organizer, I must pay for shared iCloud storage and any subscriptions that I invoke such as Apple TV+, Apple Music etc. but I can also pay for those purchase with gift cards and never have to use my debit card.