You must have in your contact that they pay for the days stated weather they come or not? How else can you budget to live? Maybe do what playffelt suggested offer a 3 day min but be prepared for them to take advantage.

What about with your part time kids (on there off days) offer a drop in play for kids of “ stay at home moms” who want there kids to get social interaction for a few hours a week.
I did that with 2 little ones and they come from 9am-12pm for $20.00 a day two times a week and they must pay there $40.00 a week regardless if they come or not. They are paying for the space and I can’t fill it otherwise. For me It’s a guaranteed extra $320.00 a month. It can help fill a gap!

Don't be a door mat tell your parents that you have a business to run! Period!