I agree with kidlove... you need to know where your "hard line" is.
I have the parents write into the contract their expected hours of care. If someone tells me 6:45-4:00, and shows up once at 4:15, I am willing to let sleeping dogs lie. Should it become a habit, I would ask "I have noticed pick-up has been a little later, do you anticipate that being a permanent thing?". As long as it is within my hours, and I am aware, then fine. But the day that they show up at 4:45 (15 min after close), with not a word of explanation, that would be it. I would remind them of my hours, and ask that it not happen again. The next time it did? Late fee, absolutely.
Having said that, I have a family returning from mat leave next month, that I will be staying "open" for an extra 15 min, the 2 days a week that they are here. I offered that, because I changed my hours in the year that they have been away. They are a later drop off, and I know that if I needed the kids picked up earlier, I just have to ask mom. She is a parent worth keeping around, and has become a friend, in addition.