I would tell her immediately that you don't have an opening for part time casual right now. She's welcome to stay of as full time otherwise you will need to start advertising to fill the space.
This is class parent thinking that their daycare provider (babysitter in their mind) is home anyways, the money is just a bonus. Not concerned at all about our budget or our pay day.
If you "wait and see" you will run into trouble. Because you've made it ok by not saying anything.
I encourage you to not allow a three day deal unless they are scheduled days and there is no make up days if she is absent for any reason.
If you do allow her to go casual then get double pay. Charge a LOT more per day.
Agreed with all the others. You might also want to consider if this child would be a fit for you part time. I actually did not renew a contract for this exact reason. DCM "told" me that this was how it was going to be now (moving from part time to casual drop in), but she didn't ask me anything about it before "telling" me this. I DO have drop ins, but they need to be over 2.5 years old, fully potty trained, and verbal. Those are strict because I don't want to transition kids again and again and again. I want them old enough to appreciate some socialization, and manage their own business, and then they can join us in our fun.
This kid had none of that and the worst separation anxiety I have ever seen. I pretty much had to increase the days when he was part time or I would have had to tell him goodbye much earlier. Your dcg might really have trouble with the sporadic casual schedule resulting in much more work for you. Take it into consideration, and charge accordingly for how much that trouble is going to cost you. But yes, please set boundaries before any of this care takes place.
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.