Hi Jamie, and welcome aboard! I was just on the Disney Magic in January for an amazing cruise to the Western Caribbean, stopping at some of the same ports that you'll experience. I enjoyed two great Port Adventures that I'd highly recommend.
In Grand Cayman, there are a number of adventures that include the Turtle Farm. The
Boatswain Adventure Marine Park would be a great one for your family with older and younger children, as there is something exciting for all ages there. You'll love being able to climb into a shallow tank with little sea turtles as they swim around you. Your 14-year old might enjoy snorkeling with one of his parents amongst some really neat sea life, while your 2-year old checks out the gators, turtles, dolphins and other creatures. I cannot tell you how much I enjoyed that experience, as did my son who joined me on that adventure.
In Cozumel, we did tour the ruins on the Cozumel Ruins and Beach Tour, and though I would recommend that for older children, due to the ruins, the beach part is perfect for all ages! The beach we spent time enjoying was Playa Mia, where the
Cozumel Beach Break Port Adventure also takes guests. This would be the perfect spot for your family. With exciting water activities such as the climbing inflatable iceberg for your 14-year old, the kids waterpark area for your little one, and of course relaxing on the beautiful beach, it suits everyone's needs! The food there is great, with kids offerings as well as traditional Mexican fare, you won't be disappointed!
I hope those suggestions help, Jamie! Please let me know if I can answer anything else for you as you continue to plan your Fantasy cruise!