Do you have a written contract? What hours have you agreed to? because I am confused. You said that they told you 6 o'clock is pick up time. But then you said they aren't getting picked up until almost 7. But you also said that you agreed to these hours? Just looking for clarification, because if the time that you AGREED UPON ORIGINALLY was 6pm.... then DO NOT feel guilty about sticking to it!!
Either way, I would also send out a notice either reminding them of the expected pick up time, or stating that effective X-date, your hours will be.