Yup late fee !!!!! Post a note on your door that effective immediately any one arriving after xxxx o'clock with have to pay a late fee of $xx /minute (whatever you decide is severe enough to stop them from doing it) in cash at time of pickup. Repeated lateness may result in immediate terrmination.
If someone is late and they don't have your cash you tell them you must have it in the morning or I won't accept your child into care. But you have to enforce it and stick to it .... Otherwise your wasting your time. I would also hand a copy of the letter to each parent saying this is a copy of rhe note posted to please read it and sign it and hand it back to you so they can't say they didn't see it.