so, your provider told you that you either need to pay for full care or she will fill your spot?
You may want to consider looking for care elsewhere then...you never know, sounds like she might be willing to let you go all together once she finds someone with more pay. If, I were her I would try to work another family around you, due to the fact that you have been there since the beginning. However daycare is a business just like any other, if she is giving you a really good deal she could have realized how much income she is losing by keeping your part time and not taking a full timer. Is there any way you can meet in the middle?
I'm assuming the issue is: you have a part time schedule but it changes sometimes? Is she charging you for the days only, no minimum required? maybe she is losing too much money due to the fact there is no schedule to rely on?