Hi again, Tina!
I do have a couple of options for you so that your family can still experience an amazing dolphin swim!
It looks like you'll be sailing on the Disney Fantasy's Western Caribbean cruise. Two of your ports offer dolphin swims or encounters for families beginning at age five or age three. Since you have a two year old as well, both of these ports also offer Port Adventures called "dolphin observers" for your husband to join along with your two year old. You'll all travel over to the dolphin swim location, and your husband and youngest son can watch from a designated area as you and your older son participate in the swim.
While that adventure will be fun in Cozumel, I'd actually recommend the dolphin encounter and the
Dolphin observer adventure (with turtle farm) in Grand Cayman instead. In Grand Cayman, the
Dolphin Encounter and Turtle Farm Adventure also includes time to visit the amazing turtle farm. This will be great because after watching you swim with the dolphins, your husband and youngest son will get to fully take part in the rest of the adventure with the turtles with you. When you book these online, add your name and older son's name to the encounter and add your husband and younger son to the observer and you'll be all set!
My family and I loved exploring the turtle farm in Grand Cayman when we were there with Disney Cruise Line earlier this year. There is even a tank where kids can get into the water with the baby sea turtles. My son loved this!
Have fun planning and please let me know if I can help with anything else before you set sail, Tina!