As inconvenient as it is you also have to look at it another way - it is money that you would not normally be making and that is a good thing. You need to plan for the possibility of child being there any given day. It sounds like parent pays only for the days she uses whether that is her scheduled days or extra days. Since you aren't able to give the days away to anyone else then the fact she picks up as many extra shifts as possible is a good thing for you as you earn more income.

I know it can be very annoying when you want to prep things in advance etc not knowing how many apple cutouts or dishes of jello you will need the next day but one of the tricks is to plan this kind of thing for the scheduled days and on the other days plan more open ended things so that the extra child is not an inconvenience.

You don't say how many children you have in care. I am guessing that you are not full and therefore there are days you assumed it would be just you and your own children free to do whatever you like only to find out in the morning the daycare child is coming and that changes the plans. When all spaces are filled and you are in daycare mode every day it will probably seem less of an issue and you will develop a pattern.

The other option is to pick one day a week such as Monday or Friday and tell mom that unless it is a scheduled day already that you are not available for call ins and that way you can use that day to go on outings, do a planned craft, etc. knowing the child won't be there.