Hey there margot92,
It sounds like you might have Family Sharing set up if you are creating an Apple ID for a child. If that's the case, you'll need to know the payment information currently set up for your Apple ID since this is set up for the entire family as well. Check your current method or update its information in Settings > your name > Media & Purchases > View Account > Manage Payments.
Once the payment is updated, try creating the child account once more. You'll do that by using these steps from your device:
On your iPhone, iPad, or iPod touch
To verify that you're an adult, you need a credit card. On your iPhone, you can also use a driver's license or state ID added to Wallet where available.
In iOS 16 or iPadOS 16 or later
1. Go to Settings > Family.
2. Tap the Add Member button
.
3. Tap Create Child Account, then tap Continue.
4. Enter the child's name and birth date. Be sure to enter the correct birth date — you can't change it later.
5. Follow the onscreen instructions to finish setting up the account. For your child's Apple ID, you can use their email address, the suggested @icloud.com address, or their Game Center nickname.
Create an Apple ID for your child - Apple Support
Take care!