Babygirl is right, the thing is you are offering her goodies ,it is great she has a good time with you, perhaps you need to take a step back and re-inforce her Moms status, be supportive of her parents and whilst Grand parenting is a good positive help in her life you are not her parent, you are supportive. When she crys to stay with you do not feed in to it that is why the parents dont like it, she is their child.
how old is she? You may be unintentionally training her to cry by offering the goodies-
maybe that is what the parents are upset about-
All transitions for little ones are hard- you have to try to stay calm and with a sweet smile tell her- "it will be ok"- do not get upset with her- do not offer goodies- stay rational and try to act happy even if you are not- see you will help set the attitude that these transitions are hard but OK and go on about your life- I know it's hard but she will get used to it.