Dentist demanding copies of adoption decrees. Is this ok? I am livid.

Thank you in advance for reading and replying. I adopted both of my grandchildren as infants, now ages 9 & 13. When I go to the doctors, hospital, dentist I am lucky to be able to know all biological parents (one being my adult mentally ill daughter) and can and do provide comprehensive family medical history etc... As a result of this medical history provided, today, I brought the kids to a relatively new-to-us dentist for the 3rd time and they stated that they required proof that I am their mother. This has never ever happened, and both children have had numerous medical issues requiring multiple specialists etc, pediatrician, other dentist and even their school have never asked for proof that I am their mother. There has been no issue by the bio parents (both fathers out of the picture since infancy, and the mother is my adult mentally ill daughter who I have a great relationship with and there are no issues). So no one has contested that I have the right to bring them to the dentist. They asked for a copy of the adoption decree. I adamantly refused. I stated that I am their mother, have been for their entire lives, and asked if they ask all parents for proof of parenthood, and stated that they are the very first EVER to ask. She tried to tell me it was the law, but I don't believe so. She also stated "since its a family adoption, we have a form you could have your daughter sign saying it's ok for you to make decisions..." WAIT WHAT??? This is very personal and private information. I do not want that court document in random files all over the place. Can they legally refuse treatment if I do not provide these court documents? Why isn't the fact that they have insurance (which already required proof of parenthood) enough? The oldest child is African American, and I am white. I hate to even bring that up, but I do wonder if it is a contributing factor to this issue. As I said..thank you in advance. They did treat my children today, but I believe they plan to refuse further treatment without proof that I am their mother.