I would mention it to the parent as well. All my parents drop off by 8am, so it's never been an issue for me until recently a parent had a few weeks of a flexible working schedule and she started dropping her child off at different times. I explained to her that she needed to have her child here prior to 8:30 because we go for a walk at that time and then play outside from 9 to 10:15. Well the next day she wasn't here and I wasn't waiting for her. Stuck a note to the door and said gone for our walk, back at 9:00am. She was here waiting for me and I reminded her of the drop off time again and she hasn't done it since.

So I would mention it to her, she probably doesn't even realize that it's an inconvenience for you. Explain that you need all the children to be at your place prior to 9am.