My pays are on Monday as well. I also have the three minimum if your days vary. Otherwise they choose the days they need and book those in the contract so if a parent needs Tues and Thurs they pay for two days (although the daily rate is more than my normal daily rate). But that is the only days they may use. I then advertise to find another client needing some or all of the remaining days. If your days vary then you pay at minimum the three day rate and it isn't about how many days you actually come it is about me holding the days available for you and not making it a shared space arrangement. So you are paying for the days to be there should you need them.

I think what you need to do is explain to the parent that they are paying the rate for the space to be there and not by the number of days used. The alternate method is to charge a weekly reservation fee for the space and then another amount for each day used. The idea being you know from the get go what the minimum amount of money you will receive is so you can budget accordingly.