I have been in MANY situations very same as yours. It starts with "may we pick up late once a month?" you let them (thinking..."once a month is no big deal!") then it goes to twice (without asking you)...you let it slide. Then somewhere along the line you realize, you are being "used" and also realize, what started as a "favor" from you (in your control) has now become and "abuse" by them and somehow THEY are now in control! All you need to do is..either let them know "face to face" that the late pick-ups can not occur anymore at all! or...you need to send out a reminder of your hours, with an addendum to your contracts stating: you will no longer allow ANY late pick-ups or early drop-offs with out an "outside hours of oporation fee" (the fee being what you choose. 5$/15 min. or a flat fee of so much) just STICK TO YOUR GUNS!!! In this job we are all "push-overs" from time to time, (thats why we are so good at what we do) we have soft hearts and are willing to do favors for these sweet kids and their parents. But you need to know your limits and if someone abuses them...money talks!!!(I am the first to take this advice, I have been a "door mat" off and on for almost 10 years)
good luck!