Sorry it took me so long to get back to you, Karen. I had major computer problems! Bought a new laptop and now I'm online again. (yeah!)
I would expect that you will have many kids in that age range on your cruise because it is Spring Break during that time. I think the So. Caribbean generally gets less kids than the Eastern/Western itineraries, but since you are going at the end of March, there should be plenty of them to keep your kids busy.
We took the Radiance to the Pac NW, so I can't answer your questions from past experience. We will be taking the Adventure in July, so maybe you'll be able to post when you return and answer some of MY questions!!
I can tell you what we PLAN to do though. In Antigua, we're still a little "up in the air". Not sure if we want to do the 3.5 hr. catamaran tour or the island tour. In Barbados, we're hiring a taxi van to take us to Harrison's Caves and then we're returning to the ship for lunch. Then we'll be off on the Jolly Roger Pirate Cruise. I've heard so many great things about this tour and it should be a lot of fun. In St. Lucia, we aren't quite sure what we want to do yet. All the tours are so long and go over such rough roads, that I'm not sure if my kids could handle all the curves without getting carsick. If you have any ideas, let me know!
Have a great cruise and let us know when you get back!