I think this is a common problem with the parent/caregiver partnership. I like what Judy suggests, but also change your contract so that parents know what to expect. When I have had this problem I have used the following analogy with parents:
"Do you phone the bank and tell them that you are going on holidays and will not be paying your mortgage or car payment. No? Then why would you consider my services to be different?"

My experience has shown that analogies help the parents to understand the situation even better.

Good Luck.