If you don't have a set number of days they can use for vacation then assume you will not be paid anything for this week. The parents likely have the entire week off and will bring the child only Mon and Tues if they need to get some stuff done - otherwise they are all on vacation.

For sure an amendment to your contract that gives only a specified number of no pay vacation days. Alternate idea is to charge half fees with the idea that you still have expenses for the house utilities, or part of can of food opened for the group whether they are there or not but recognizes the less work load and unused expendable items like craft supplies. It also means you can budget to never be less than half fee in income so if you normally charge $200 a week you can budget that you will never receive less than $100 a week.