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.