I tell them that if they have an appointment scheduled for the afternoon, they need to pick up the child before naptime which starts at 12 and like BC, only one pick up/drop off per day, so they cannot return. I expect them to come before 9 a.m. and to be consistent in their drop off time for the reasons CatHerder said. I don't "do" adults well in the morning!
They don't get breakfast if they're not here by 8:30, but if it's an unusual circumstance, I will give the child something to eat. If they notify me in advance, they can come later than 9, but I expect the child to be on our schedule (fed, ready to play, ready for nap at usual time).
You can't deny a parent their child if they come early to pick up, but you can definitely make policies that enforce respect for the group, but also allow for individual needs and circumstance.