Can't update my daughter's birthday in family account and she is 18 but still has a child account.

6 years ago I set my daughter up with a child account. She was 12. Unfortunately, each year her birthday has not updated and her account still says she is 12. I have unsuccessfully tried everything to change her birthdate. The child account is very limited, and is causing issues now that she's older. Is there anyway to officially change her information? It never saves when I do it in settings, and just reverts back.