I was going to suggest she pay a set fee for the month to reserve her space in your care - and then a daily fee for any day she uses. So it is like a reservation fee plus a usage fee. If the weeks over time balance out then she will pay more than your full time rate some months and less on others but over time it will be the same. ie even in the busy months she still needs to pay the daily fee not drop down to the full time rate. If she wants that she needs to book and pay that every week as in pick one system of payment and then stick with it. The set fee part will pay for the child's basics and allow you to budget and then anything else you get from the child is bonus. You could also ask for a monthly calendar and figure out a basic average number of days and ask her to pay that on a weekly basis so it spreads out the fee. How many hours she uses per day doesn't count so don't go to an hourly rate - if she brings the kids it is a used day and she pays accordingly as the space is gone and can't be given to another child.