Cruise critic has tons of stuff that you can read about Key West and some of the places to go. Depends on what you would like to see and interests of your kids. I would like to take advantage of some of the educational things but disguising it with some fun! Here's one place that I think we'll go to : Its inexpensive, and great for military/ history buffs and has a place to snorkel! Its not on the trolley route but would be a nice walk from the pier.
http://www.fortzacharytaylor.com/home.html I also read on cruise critic that you can hire a taxi minivan for about $35 for 1 hour and do your own tour. I couldn't find any place to do this ahead of time - guess when you get there. Its good if you know exactly where you want to go, like make a stop for your obligatory photo at the Southernmost point, then drop you off at your destination where you want to spend the most time. Also, parasiling is offered leaving right near where the ship docks for about $40 pp. I like being stateside for this and close to a major US hospital in case the unforseen happens (that's the ER/trauma nurse in me speaking!)
We went to the library yesterday to check out a few travel books on Florida and the Keys as well as some Hemmingway books. OK, OK, I'm such a mean mom but I want my kids to be familiar with the author and his writings if we go visit his home. We are big cat lovers (have 3 at home) and of course their fascination is to see the cats with 6?? toes!!!