Hey kornelia80,
Have you tried to update the payment information which is associated with your Apple ID? You can add a new payment method, then remove the older one. Since you are the Family Sharing organizer, you always need to have a payment method on file, which why you need to add a new one first.
Add a payment method to your Apple ID - Apple Support
Then, this page has how to remove a payment method:
Remove a payment method from your Apple ID - Apple Support
Then, you can try to create a child's Apple ID again.
We hope this helps!
Sincerely.