Ahoy, Karen!
While nothing can compare to the custom-built cruise terminal that exists at Port Canaveral, Florida, keep in mind that you will be boarding a Disney Cruise Line ship!
For our first few cruises, our family embarked from Port Canaveral. Just walking into that grand terminal is an experience, isn't it? Two years ago, we tried our first cruise from Miami. The terminal there is different. It is much smaller, so there isn't the vastness that you experience in Port Canaveral. There also is not a model of the ship, but there is still a character meet and greet area. When you walk through the hall to board, you still experience the photographers with a backdrop, and the walk over the gang plank to the ship.
Returning your rental car in Miami is just as easy as doing it at Port Canaveral. I might suggest that one adult drop the other off with any children in the party and have the porters take all of your luggage. Then, while one person drops off the rental car, the other can get inside the cruise terminal with the kids. While you won't be able to check-in until everyone has arrived, it is much easier to get just one person with no luggage back to the terminal on the shuttle. We have done this at both PC and Miami with no issues.
Once onboard, you will be engulfed by the Disney Cruise Line magic all over again. From your family being announced (did you know that you can choose how your family is announced?) to the crew clapping for your arrival, your family will feel like royalty. I recommend heading to Carioca's for a bite to eat as soon as you are onboard the Disney Magic. It is a buffet on embarkation day, like Cabanas, but I have found it to be slightly less hectic.
I know you will have a MAGICal time on the Magic out of Miami and at Mickey's Not So Scary Halloween Party! Please visit us again here with more questions before your family sets sail next month.
Happy Cruising, Tracy V.