Welcome aboard, Jeannine! I'm so happy to hear you've booked a Western Caribbean cruise with Disney for next summer! There is so much to look forward to!
I think that the best snorkeling in the Western Caribbean is in Grand Cayman, and of course, Disney offers quite a few options for snorkeling Port Adventures there. Two that stand out to me for your family, including your five-year old, are
Shipwreck and Reef Snorkeling and
Catamaran, Reef and Kittiwake Snorkeling. Both visit a shipwreck and another stop at a shallow reef. However, if you want to add even more fun to the experience, you might consider the
Nautilus Undersea Tour and Reef Snorkel. First, your family gets to ride a semi-sub to view the beautiful undersea world. Then, you'll enjoy snorkeling time in the reef.
Take a look at all of the Port Adventures, and once you choose a port, be sure to scroll down on the left and check the Age Range of 5-7, so you're only viewing those adventures in which your child can take part.
Don't forget that Castaway Cay has it's own
Snorkel Lagoon (you can easily rent equipment on the island), where Disney has hidden some fun surprises to spot under the water. Keep your eyes peeled!
Let me know if I can help with anything else as you continue to plan, Jeannine!