we cruised out of miami last february and ended up staying in the bayside area -- we found cab's to be very easy and affordable -- around $25 to our hotel, ~$10 from the hotel to port and from port back to MIA was a flat rate, seems like it was 20 or 25 $.
I know i'm not answering your question, but i did want to mention it, as the airport area didn't have a lot of activities for the kids (or adults) -- bayside had a lot of stuff you could walk to and we really enjoyed getting out after the travel. As a bonus some hotels have port views and you may get lucky and get a view of the ship the morning you depart!