I think you just need to clarify that you are available Mon-Fri for care but that you require 48hrs (or what ever you specify) notice to be there for the day. With less than 48hrs they can request but you cannot guarantee you will be able to provide care as you may have made plans for that day based on the understanding you had no children in your care for the day. If they NEED last minute, unpredictable Mon-Fri availability then they NEED to pay full time care whether or not they are in attendence.