child apple id

Setting up a child account has to be done under Family Sharing (Create an Apple ID for your child - Apple Support.) So yes, if you do this you will also have to add your payment method as a guarantee for other purchases (see the longer version of this below). There are ways to limit this such as setting up Ask To Buy on the child's account. Approve what kids buy with Ask to Buy - Apple Support

You can transfer a child to another account later. See Move a child to another group using Family Sharing - Apple Support

From: How to share purchases with your family - How to share purchases with your family - Apple Support

"When you turn on purchase sharing, everyone in your family gets access to apps, music, movies, TV shows, and books that family members buy. The family organizer is billed for family members' purchases."

Ideally the second sentence would also mention that others can still pay for their own items, they just have to use personal account balance to do so (it is mentioned if you go to the tiny footnote near the bottom of the page and follow the link there). Turning on purchase sharing simply activates the feature whereby if a family member does not have adequate personal balance to pay for something then the organizer's payment method (e.g., credit card) will be charged.

Read the document for the link to "learn how purchases are billed if a family member has Apple ID balance". This tells you how it works. In essence 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 it cannot bill the primary payment method, the Organizer will need to make another payment method the primary 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 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