I don't offer care for kids wanting less than 3 days per week.
I have occassionally done drop in care for families who used to be enrolled but are no longer but ONLY if they pre-arrange it, pre-pay for it and if I have the space to accommodate them. So if they haven't pre-approved it (and pre-paid for it) I don't wait around for anyone.
I would do as DBug suggested and notify them in a written form. This way they have time to either commit to more use (if you have the space) or to find alternate care arrangements.