The beaches at Key West are not very nice. My family spent 10 days in the Keys in 1998 and could not find a single beach worth our time. The sand is rocky, and the amenities are few and far between. The water is great, and the snorkling was great, but we went to the snorkling spots by boat. If your family likes beaches I would recomend getting sand between your toes at any of the other Western ports of call. There are a lot of other things to do in Key West, and I'm sure you will have a great time there. But you may want to pass on the beach thing.
Hope this helps. Twinave