That poor baby
IMO parents should be held accountable. I know that with most obese children it has a lot to do with genetics, along with diet and exercise or lack thereof. But a child that young can't drive to the restuarants or purchase her own food. It is such a sad situation. Maybe when school is out invite her to spend a few days a week with you. You and your DH could get her started exercising and eating healthier.
It will be hard if her parents and grandparents aren't trying to help her. {{hugs}} to you for trying to change her life and her future.