Q: family sharing with multiple credit cards

To have your personal purchases on your personal credit card and not the family purchases, you could set up the other accounts with a gift card that gives them store credit.    See the article:

Family purchases and payments - Apple Support

Especially the section:

Making purchases

After you set up your family, any time a family member initiates a new purchase it will be billed directly to your account, unless that family member has gift or store credit. First, their store credit will be used to pay the partial or total bill. The remainder will bill to the family organizer's card

I also meant to include this link:

Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support

See this section:

Payment methods for Family Sharing

If you're using Family Sharing and you're the family organizer, you can use the following payment methods:

• Credit cards
• Debit cards
• China UnionPay
• Mobile phone billing

You can also create an account for a child under 13² using these payment methods, with the exception of mobile phone billing. (You can only create a child account with a post-paid mobile phone billing account.) After you create the child’s Apple ID, you can change your payment method to a different method, such as a China UnionPay debit card or a pre-paid mobile phone billing account.