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.