When you filled the other days of the week did you let this family know that from now on you would not be able to allow her to switch days due to the fact there were no longer open days. It almost sounds like this is the first the daycare parent is hearing that she can't switch and is doubting you thinking you are doing it just to get money for days they aren't using. Show her your schedule of kids in care to show that there are no longer any open days reminding her that she had the option to take the space full time to reserve the days but chose to only sign up for some of the days and that left you having to find someone interested in the other days and now that you have she is confined to only the days she selected. Rather than get into a back and forth arguing/blaming game your schedule and the rules of how many kids can be in care on any given day is the end of the story.