I had a parent do this, except a few times it was 20 minutes early!! Most times it was 10 minutes before their scheduled drop off time. They were my first client of the day, at 7am, the others came at 7:30. So some days, I was brushing my teeth when they would ring the doorbell, or still tidying up, getting ready for the day. It irked me SOOO much!! And I always like to open the door for them, cause they were the type to ring the bell instead of knockingl when my kid and husband were still sleeping!!) So I would see them pull up and open the door for them. Them coming early meant that they woudl ring the bell and sure enough, my kid would be up. Back then, my son would sleep till 8:30am, so him getting up at 7am meant a long morning for me with his crankyness.
I ended up canning them since they were flexible part time and I needed to fill with a full time spot. But this was among some of the many reason why I loved seeing them go!!!

Any new client who have asked for a 7am drop-off, I straight out tell them," please make sure its right at 7, and not before, I have limited time in the morning and I'd like to be ready for when you come." One current client kept coming early, but would stop at Timmies for a coffee and wait till 7 to bring her son over. She later found that the clock in her kitchen was fast and that why she always has her son ready to go earlier than normal time. But at least she had the respect to not bring him until 7am on the dot. It does make a huge difference if you're anything liek me. I cherish those 15 minutes I can eat my breakfast and enjoy my coffee and news before they come.

Maybe the next time you are talking, ask if they use their cell phone for the time in the morning, casually say that a few time they have came early and you hadn't quite had your coffee, or the bed made yet or whatever, maybe they can get the hint.