I have also come across this issue over the past couple of months and I am revising my contract as we speak. My parent would advise me at the time where payment was due (Monday prior for the following two weeks) that her child wouldn't be here a day or two. I thought the first couple of times would be it but it seems like she continuously does this and I am not able to replace the income on such short notice.
I definately need to revise my contract and send it out to all parents so I stop getting burned like this.