FLORIDA KEYS resident or expert?

I am taking a vacation in a couple of weeks and would like to go to the Florida Keys..
I am seeking advice on a safe and child oriented location to go.
I have a 6yr old, 10yr old and 15yr old so it really has to be family based area with lots of activities for kids.
I am aware of the amusement parks, Im trying to get away from that and do something a little less high strung..
Boats tours, seashells, snorkeling, fishing, stuff like that.. but it has to be in the Keys area…
Hope someone can help… Thank You