Around here that seem to always be people looking for part time care. First I would advertise the open days and see what comes up. You could also ask the family to switch days or leave. A lot of government workers here have the option of working longer hours and getting fridays off so many asking for a 4 day week. That way they could still share. I almost always have at least one mom on mat leave and they know that they will need to juggle the days the older child comes to fit in with what is available.

At minimum I assume they are paying for the full day at least. Their daily rate plus two three day part timers would equal the regular full rate plus still give you an extra day to add someone or to have an easier day.