Do you have legal AND physical custody, or legal custody only? And is it temporary or permanent? Also, where's the child's father? Can't he/anyone else (yours/his family members) supervise visitation?
If she wants to visit with her child and your providing an avenue that she may not be able to overcome (paid visitation) the you're hurting your grandchild more than helping her, for she will ask questions when of age and will get the right answers.
You should be able to get unpaid supervision for visitation purposes, so as to enable your granddaughter to see her mother.
Parents-in-Law. What do/did you call them?