I've done shift work in the past, and there is no way that these parents don't know more than one weekend in advance what their week is going to look like. I agree with previous posters that you need to make up a contract, and I totally sympathize with you on finding it hard to present the daycare parents with that! I am just starting out and I'm already nervous about putting my foot down, afraid they will decide to go elsewhere. But you know what? You deserve better than this. And if you continue to put up with it, you are going to be miserable, and ultimately burn out. And it's not worth your health or your sanity.
Let us know how things turn out!