you really should sit down with them and see if they'll sign her over
speaking of which is she there today?
I also think you should be writing a 'diary' ....document what days she is there, what condition she shows up in, what activities you teach her or behaviors she learned elsewhere, what items (including food and drink) you have to provide for her. Having timelines and records can help you later if the so called 'parents' get nasty about anything and start making false accusations or wild claims. They are getting away with a lot now but it will get a lot harder to continue their ways when school comes into the picture. When authorities come down on them they will immediately try to shift blame and you will be the likely target.
Sorry to be gloomy and negative but I have seen it happen....lazy 'parents' are never to blame and you know they would never admit their own faults.