I have a rule in my contract that morning dropoff must occur before 9am and afternoon pickup must occur after 3pm. This is because we go on field trips to parks shortly after 9am and I'm not waiting around for anybody and beause the children's naps go until close to 3pm and I'm protecting all their naptimes.

You have to make up your rules and have good solid reasons for them and put them in your contract and don't let anybody break the rules! If I had an afternoon child I would insist that they arrive by 12:00 if naptime was at 12:30 or 1:00pm because that would give the child a little time to play before being stuck into a bed.