What bothers me about your situation is not just the money. It's the attitude this dcMom seems to have that she's in charge and you are her employee. That is WRONG! I agree that you have to put your foot down and talk to the dcMom now, but explain that you must be able to budget, then decide how much to charge using the advice given here. I'm sure you will charge what is best for you.

However, the sporadic days would bother me more than the money. I plan my weeks and we go on a lot of trips to the parks and different places. I want to know how many children I'm feeding when I plan the meals the night before. I've had part-timers who I insisted came 3 days/week minimum in the past but I wanted their schedule a month in advance so I could plan. That's just me. If you don't mind being a drop-in centre that's different. But decide what is ok with you so your business runs smoothly and remember that you are in charge of your business.