In my opinion as long as they show up before a set time (that is clearly noted, usually morning snack) they are free to come when they come.

BUT it is your daycare and if it isn't working for you then make it known what your expectations are. If they had asked you open early and are now coming late then I would make it clear you will no longer open early as they have inconvenienced you and are clearly not in need of it.

Do you actually talk to every parent for 5min in the morning? I would find that the kids that are prone to crying would just cry more if you stood and talked to their parents you are delaying their quick exit and making it more difficult for the child! Here, the children that cry (usually during transition) do best with a very quick hand to hand pass off.