This woman is treating you like a nanny. You provide GROUP care which means you have a set schedule and a routine and everyone does what is on your schedule at the same time. If a kid comes after snack time then they miss out on snack. If you do not serve breakfast or serve it earlier than someone's arrival time then they do not get breakfast.

You need to put your foot down with this lady. She is treating you like her personal nanny. It doesn't work that way.