My son has done many western Caribbean cruises. His first was at the age of 20 months, and his last was in June 2005 at the age of 11. I think he has done the western run 7 or 8 times in total. It is a good itinerary for kids. However, I wonder about kids as young as yours for the Stingray City tour. It may be too overwhelming for them. Chankanaab is supposed to be re-opened by the time of your cruise. This is a fine place for kids!
Let me know if you have more specific questions! Cruising the Caribbean with a youngster is a specialty of mine!