You also can receive Priority Check-In if you are a Platinum member of the Crown and Anchor club. (5 or more Royal Caribbean cruises)
Otherwise, I don't think you can purchase priority check-in. If you have very small children you can speak with a pier attendant and "perhaps" they can arrange for you to be helped out but I can't guarantee it.
I would suggest arriving at the port around 11:00 a.m. this should minimize the amount of waiting time you have before you are on board.