Welcome aboard, Mike! Grand Cayman is one of my favorite Western Caribbean ports. My family has had many magical visits there with Disney Cruise Line and we've experienced some really fun Port Adventures on the island.
The Dolphin Extreme Swim and Turtle Farm is a spectacular adventure! My daughter had the time of her life when she and I swam with those gorgeous creatures and got to interact with them in the water. This adventure is even more magical because your family will have the opportunity to enjoy the Turtle Farm across the street as well. My son thought it was so exciting to see the huge sea turtles and even get in the tank to interact with the baby sea turtles. Plus, I had the best conch burger there! Do try it if you get the chance!
I'm sorry to say that the age restrictions with Disney Cruise Line's Port Adventures are firm and do not include any wiggle room. When you go to book, the system will not even allow a child under that stated age to be added to the adventure, and they cannot make exceptions, I'm sorry. We ran into that problem a few times as my son is mature and tall for his age, but still couldn't join us for specific adventures until he turned a certain age. However, the good news is that your whole family can still enjoy the day together because one adult and the other children can swim with the dolphins while another adult and your youngest can join in on the fun, still travel with you, watch the dolphin swim, and then join you for more fun together at the Turtle Farm. Simply book those two on the
Dolphin Observer and Turtle Farm adventure and you'll be all set (plus you'll save quite a bit of money on their adventure cost). I understand it's not the best, but at least the whole family can still enjoy the day together.
You're in for such fun, Mike! Enjoy your cruise!