I certainly would have it written into her care plan. Everything no matter how big or small is best to be written down.
If she has a problem brushing her teeth meaning she is non compliant I would do a reward chart so that the carers don’t have to spend ages prompting her to let them brush her teeth.
My son is 21 and we still have to do these. He loves getting that special something at the end. It could be 14 times brushing teeth in a week.
I don’t know anything about your daughter but if she is having such problems that she is losing teeth then she must be suffering and that’s not acceptable in any situation.
If she has a social worker I’d be on the case with her after I’d written to the care home about what you require for your daughter.
I’d get her checked thoroughly by the dentist and make sure she has a programme from them that has to be followed. I would hate to think that her teeth were so bad that any sensitive teeth would leave her in such discomfort that it would be unbearable.
If no joy go down the route of a formal complaint, medical, even local MP but she must be in a lot of pain and it’s been a long time.
I’m sure poor oral hygiene can lead to other problems it’s best to Google.