Well first of all you need to write up a letter for all parents saying that they will be required to pay for all days contracted for weather they attend or not. In the letter have an area (graph or something ) with the days if the week and what each was originally asking for

Dear parents

Starting Monday jan 6 you will be required to pay for all days originally contracted for regardless of attendance so that I may schedule accordingly. Also all fees are required biweekly on Fridays before 5 pm for the following two weeks care. The days that I have you scheduled for are as follows:
Monday xxxxx
Tuesday xxxxx
Wednesday xxxx
Thursday. Xxxx
Friday xxxxx

The total amount due biweekly is. Xxxxx
Please sign and return this form by Friday dec 13, any form not signed and returned by this date will signify that care is no longer required.
Thank you for your understanding
Your daycare provider

This will let you know who really wants care and who is just looking for a back up. Unless you want to be a drop in centre you need to make people pay for what they sign up for.