Ahhh, Key West. I feel obligated to talk a bit about her here. Key West is not a city that lends itself well to "excursions" or blanket tours. Key West is an eclectic, fun, interesting city that must be experienced in a different way.
Right up front, my VERY FAVORITE thing to do in Key West. Every night, in a place called Mallory Square, right next to the cruise ship dock, there is a "sunset party". Twenty or so arts and crafts vendors (who must apply and be selected) display their wares. There are steet vendors and performers of all varieties. When we were there this time, we saw a man juggling fiery torches on an eight foot high unicycle, a man blowing thousands of bubbles with various contraptions, a man with performing cats, a living statue, a guitarist, and many others. One performer had a dog who collected his tips from the crowd and put them in a hat. This was literally the highlight of my son's trip! He thought it was great. There are face painters, balloon artists, henna tattooists, and so much more. I LOVE, LOVE, LOVE Mallory Square at sunset, and I wouldn't have missed it! It starts up about an hour to forty five minutes before sundown. This trip, it was sorta 5:15 - 6:30. I'll post a few pics on facebook.
Visiting Key West from a cruise ship, with a limited schedule, is difficult. I will say, it's probably not a brilliant cruise ship stop for kids. Rather than try to dabble in it on foot from the cruise ship port, I would immediately hop in a cab and get away from the port area. As an adult, my favorite spot to visit quickly is Ernest Hemingway's house. It's a beautiful old home, and his is an interesting story. It's also great fun to see some of the six-toed cats wandering around. There is a very pretty lighthouse and museum right across the street from his house. Beyond that, it's not a long walk to the Southernmost Point in the US, a great photo op.
And, once you get into Key West proper, further south on Duval Street, there are a lot of shops with local art and color, and plenty of fabulous bars and restaurants. Not the tourist-trappy, cheap stuff near the port. In short, go about halfway down Duval street, and then walk from there to the Southernmost Point. Then, take a cab back to the ship.
However, near the port area, there is the Wreckers Museum and the Aquarium. I can't speak to the aquarium, as I've not seen it. However, this trip we did go to the Wreckers museum and found it very interesting. The kids were kinda bored, even with the guides doing some acting and hamming. But, I really found it told me a lot about the history and foundation of Key West. Key West was built on the wealth of legal salvaging of shipwrecks on the surrounding reefs. My seven year old son found it somewhat interesting, and he enjoyed trying to lift the real silver bar on display. I liked the crow's nest view of the city.
In short, I love Key West. It's an American city, not scary to venture out on your own. Don't write her off too soon!