Oh my goodness, the best thing that could happen to you would be if that family doesn't come back. I know you said you are a very casual daycare but the simple fact is that child is the boss at that house. That's just wrong. She's going to grow up to be an out of control teenager and selfish woman.
I know because my children are grown and there were many days they hated me when they were children and teenagers. It's because I had rules and discipline. I was a PARENT! There were not allowed to run the house. NOW that they are grown we are all best friends, but you can't let your little child think of you as a friend, you must be a parent. It's as simple as that.
Continue with your rules at your home and daycare and your children will turn out great VenusMama