As for excursions, it depends on what you are wanting to do. I know DH wants to do the scuba excursion on St. Lucia. DD and I want to do the island tour that takes you to the drive through volcano! There will likely be some things we just won't take DD with us on. It will depend on whether or not she makes any friends and has someone to stay with in the clubs I suspect. We wandered Key West on our own last time and will likely do the same this trip. Have to stop at the winery and the store that sells all the hot sauces again. Never been to the other islands so I have no idea what might or might not be fun, with or without kids.
Either way, if several of us want to go on the same excursions we should coordinate times, etc as I know some excursions have multiple times during the day.
At some point we will also want to coordinate linking reservations so we can sit together for dinner, etc.