When my child made an in-app purchase with gift card balance, it charged my credit card as well as his gift card. Why?

Additional information provided by Apple in How to share apps and purchases with your family - Apple Support

Read the document for the link to "learn how purchases are billed if a family member has Apple ID balance" ( How apps, content, and subscriptions from Apple are billed - Apple Support ). This tells you how it works. Essentially the payment method for Family Sharing is automatic:

1. If anybody in the family buys or subscribes to something, Apple first attempts to charge the item to that individual's Apple Account (Apple ID) balance, if any.

2. If a family member does not have enough personal Apple Account balance, any excess will be charged to the Family Organizer's primary payment method (usually a card of some kind). The Organizer's personal balance will not be used for purchases made by other family members. If Apple cannot bill the primary payment method, the Organizer will need to make another payment method the primary method. With Family Sharing the Organizer always has to have at least one functioning payment method.

Also note: "Some purchases, including gifts, can't be billed to Apple ID balance and will be charged to the family organizer's payment method." "Some subscriptions might not be charged to Apple ID balance."

Ref:

- Family purchases and payments - How to share apps and purchases with your family - Apple Support

- How apps, content, and subscriptions from Apple are billed - How apps, content, and subscriptions from Apple are billed - Apple Support

- Check your Apple ID balance - Check your Apple Account balance - Apple Support

- Add a payment method to your Apple ID - Add a payment method to your Apple ID - Apple Support

Adding funds to your Apple ID balance:

- How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

- Add money to your Apple Account balance - Apple Support

Purchases will use any available balance left from the gift card (or account balance) first and the remainder will go to your credit card. You may see an email for charges on both, but it should only be the portion left from the account, followed by the rest that is on your credit card.