Mine all come and go at different times too and personally, I wouldn't want them to all come and go at the same time or pick-up drop-off would be too chaotic! They arrive within an hour in the morning and leave within an hour in the afternoon...I don't plan activities during those times and it works out well. Parents keep it brief as they know I have to be with the other kids and cannot stand at the door chatting (I can see my playroom from my front entrance, but the kids often cry when I go up to see someone off, so it keeps it short.)
As for changing your contract, I would change it in increments. I don't think it is reasonable to suddenly add a bunch of vacation, sick days, etc plus a pay increase all at once. Maybe you could up the pay by a dollar or two and add stat holidays to start with. Then next year, increase again. Any new families, start out with what you want...chances are they will accept it as long as it is within the norms of what is happening in your area.