Many are pricey, you can save money by booking indepent operators or directly rather than through the ship.
In Grand Cayman you might have a look at
Grand Cayman Cruise Excurions as they have reduced rates and tell you which excursions have age and other requirements.
I think you could do the beach horseback riding and ride with your 4 year old, stingray city, Atlantis, you could rent a car and visit rum point, great family beach, so many to choose from....and thats just one location....
You can find the same type of site for different locations, much better deals and great operators with smaller groups or unique trips. enjoy