I agree. Nip this in the bud. Why do parents continue to take advantage of our kindness? People are so self centred nowadays. You give an inch they take a foot. Send out the notice and remind them of your agreed hours and how you understand how time is of the essence to EVERYONE and you need that few minutes yourself.

If it continues, turns ALL your lights off, keep your curtains closed and door locked. I recently had to do this with one family. I agreed to 5 mins earlier, they arrived on morning 20 minutes early and I answered the door after that I am closed until opening time or our AGREED upon arrival time.

Good luck! I hate this kind of stuff.