Hey Holly!
Welcome to the Moms Panel! You have made a wonderful choice in choosing the spectacular Disney Fantasy for your family's vacation! From the moment you board the Fantasy, you will be amazed at how Disney created such an elegant and whimsical vessel! I have a feeling it will be a week filled with magic and pixie dust for your family!
I understand your concern about not having a car seat when going ashore at the various ports of call. Unfortunately, the islands do not have the same transportation safety laws as in the United States. Disney Cruise Line is careful about the tour operators they choose so if you do choose to book a Port Adventure know that safety is a major consideration.
Maagens Bay Beach Break (St. Thomas) and
French Riviera Beach Rendezvous (St. Maarten) are two you might consider.
In St. Thomas you will have to use a taxi service if you want to spend the day at the beach. You will find only shopping within walking distance of the pier. In St. Maarten, there is a beach area along with dining and shopping close by. Great Bay Beach is just a short walk down from the pier.
If you chose not to venture ashore, remember there will be plenty of activities, food and entertainment on the ship even though it is in port! Your Personal Navigator, the daily cruise newsletter, will keep you up-to-date on all the ship's happenings! There is nothing wrong with enjoying the ship as your destination!
I hope my answer helps with your planning! Please write back if you need any more assistance.
Have a fantastical day!
Nancy