I allowed a parent to pay me late a day or 2 as well once. I didn't say anything and it became habit. When I did get to a point where I did say something it was complete foreign to them and I did sound like miss bitch cause I never enforced the rules to begin with. So, send out a nice email or send home a nice letter if you have trouble with the face to face. Just let her know that you understand things come up, but this is your income and this is your payday and it makes it hard to purchase crafts, food, things for daycare when payment is late. You appreciate her understanding blah blah blah.

The next time she's late I would send a memo advising her of a late fee and the possibility of termination, because at that point she's just being rude.