are you flying or driving to Miami ? if you are flying into Miami airport and will arrive at 0900, I think the safe bet (plus the kids will still be very excited about the cruise) is to go to the ship, knowing you won't be able to board untill about 1100..you also might pick up a board game entitled "Cruise IQ" that everyone can play while they are waiting to board..people watching( those getting off the ship) should occupy your time
Boy - arriving at 7:00 a.m.? This is not a problem I have heard of before.
I am sure there is some time at the pier when you are able to drop off your luggage for safe keeping. In fact, passengers leaving the ship often want to check their luggage for the day as well, so there should be some way to check your luggage early.
We took a redeye from the west coast to Ft. Lauderdale a few years ago. We didn't have the option of dropping off our luggage. We rented a cheap car, stowed the luggage, went out to breakfast, tooled around a bit, dropped off the car and they shuttled us to the ship.
you might want to see if your cruise line offers any pre-cruise excursions. If they do, then all you have to do is grab your bags, there will be a cruise line rep at baggage claim who will guide you to the bus, they load your bags onto the bus and the next time you see your bags is in your stateroom.
Another option would be to book a day room at a hotel near the pier. Find a hotel that has shuttle service from airport to hotel from hotel to pier. If you did this, you could relax, get some breakfast, go for a swim, and then head over to the pier at your leisure.
Arriving in Miami at 7 a.m. doesn't necesarily mean you will be off the plane then. Look at it this way:
7:00 plane touches down
7:05 taxi to gate
7:15 maybe you are walking off the plane then
7:30 you've rounded up everyone from the bathroom etc.
8:00 arrive at luggage carousel, after window shopping along the way & stopping for coffee (flying with kids & how early you got up to make that flight, I assume you will need caffine)
8:30 hopefully your luggage is off the plane by then.
You can have breakfast at the AP; probably a little pricy for bad food or you can take a cab to the pier. It will be at least 9:00 a.m.
You can probably get the stevedores to take the big luggage; tip the cabbie to stick around. Hopefully you've pack light carry ons that are small enough to take with you. Get back in teh cab & go out to breakfast in Miami beach. When you are finished with you long, lesiurely breakfast, you can slowly head back to the pier no earlier than 10:30 or the Bayside Market place (a big open air mall near the pier) then on to the pier. This is all contingent on being able to manage your carry ons.