Hi Jeanne,
My wife and I just signed up for the August 23rd cruise. It will be our 3rd cruise with Carnival. Have you ever cruised before? Carnival has some awesome programs for kids. We did a cruise 3 years ago with our 4 kids. At the time they were 18, 16, 13 and 11. We could have gone pretty much the entire cruise without seeing them, so we had to make a point of planning in family time. If you have any questions, please let me know and we can offer our advice (something you will never lack for on this website).
I heard doing the zipline is great. The cave tubing can be intimidating to some, especially if you have any phobias of closed in spaces. But I heard it's great and my wife and I are thinking about doing it. As far as Grand Cayman and Roatan, have you thought about doing some sort of snorkeling with your kids? My kids loved it a bunch and it's a good family activity. Roatan is supposed to have some great snorkeling.
Hope that helps.