Re: Caribbean advice
If easy air travel from the U.S. is a top priority, then I would suggest Puerto Rico as the best, USVI second, and Antigua third.
First off, it appears that if they fly from a Wyoming airport they will have to connect through Denver, so it might be best if they catch the flight in Denver. My relatives in Cheyenne pretty much always drive to Denver before flying anywhere, but then I also have lots of relatives in Denver, so the Cheyenne contingent can always find family to visit and stay with if they want.
There are lots of flights to Puerto Rico from Denver with only one change, usually in Miami. St. Thomas, USVI can also be gotten to with only one change, but there aren't as many flights and they're a little more expensive. Antigua can also be gotten to with only one change, but there are even fewer flights than for STT and they are even more expensive.
The other suggestions are beautiful places to visit, but they will all require the grandkids to make even more plane changes, in small airports, where I, personally, would be concerned about the assistance and attention they would get from the airport personnel.