I accidentally set my child up as an adult. How do I change her to a child for security, family share, billing and buying?

See the following for instructions: Update the birth date associated with your Apple ID - Apple Support

Also note the following from that same link:

To change your birth date to any age below 13*, you must be a member of a Family Sharing group, and your family organizer must approve the change. After updating your birth date, follow the onscreen steps to send an email request to your family organizer. Your family organizer can sign in to their Apple ID account page to confirm the birth date change and agree to the Parent Privacy Disclosure agreement by entering the requested information from their account payment method.