Ahoy, Dawn!
You are going to have so much fun on the 7-Night Western Caribbean cruise on the Disney Fantasy! This cruise includes my favorite port of George Town, Grand Cayman. My girls were 7 & 10 the first time I took them there, just like your grandchildren!
I would highly recommend the "Island Tour and Snorkeling with Stingrays" port adventure in Grand Cayman. Our family participated in this a few years ago, and we all still talk about it! After a brief driving tour and a quick stop at "Hell" (a rock formation where you can get a postcard postmarked from "Hell"), your family will find themselves at the Cayman Islands Turtle Farm. You might be lucky enough to hold a baby sea turtle! Afterward, you will take a boat out to the Stingray Sandbar and interact with wild stingrays! It is amazing!
In Cozumel, we swam with dolphins on the "Dolphin Push, Pull and Swim" port adventure. My daughters had the biggest smiles on their faces when those dolphins were swimming right next to them! One thing to note, you are not allowed to wear jewelry or sunscreen on this Port Adventure. Plan to lock your jewelry in your safe in your stateroom.
Our family has not been to Falmouth, Jamaica, so I do not have a recommendation for that port. At Castaway Cay, I suggest you find a nice beach chair under an umbrella and enjoy the white sand beach and crystal clear water, perhaps with a frozen drink in hand. If you are feeling adventurous, our family has enjoyed Parasailing high above Castaway Cay and our Disney Cruise ship, and it was wonderful!
I hope this helps with your Port Adventure planning!
Happy Cruising, Tracy V.