Welcome, Molina87iridie!
Thanks for using Apple Support Communities! From your post, it sounds like you're unable to add your payment method which is preventing you from being able to add a child account on your iPad. We'd be happy to point you in the right direction.
First, we recommend checking the resource below to make sure you're using a payment method that's accepted in your region:
Payment methods that you can use with your Apple ID - Apple Support
If you're using a compatible payment method, but receive an error when adding your payment, it's possible there's a balance or something else going on with your account. Please refer to the guidance below to learn how to correct payment errors:
If your payment method is declined in the App Store or iTunes Store - Apple Support
Should the steps above not help, we recommend testing to see if you can add a different payment method just in case the current method is not compatible or accepted.
Feel free to reply with the results for continued support.
Kind regards.