I remember you posting about this little one and how stressed out you were with her and seems as though nothing has changed there.
You have to make the choice if you want to continue working with her on her behavior ~ are the parents on the same page in regards to working with her behavior? Fruitloop is right, she needs to be on your schedule and are the parents going to help HER (and you) by putting her on the same schedule at home. If they aren't, best to end care now.
I had a family just recently who wanted their little one on two naps-and I couldn't accomidate and it was important to them and not willing to work with me.I tried everything to work with them. It's SO important to have the parents on the same page as you, your working together to help their daughter and if they can't then let them go .....
I get the impression that you already know it's not the right fit but so hard to say it's not working. Some times it's not the right fit for one reason or another. ((Hugs))