This is a top pick and it is great with kids. We are Coming to Marathon for four nights in June, with our two children. This answers the question, Are there any good places to take kids. Yes, One of the unique places for nature is the Turtle Hospital at mile marker 48.
They have a great focus and offer Guided Educational Tours that are fantastic for children to see. These tour experiences are open 7 days a week. They do them in groups, so you have to call and reserve a spot. It is excellent, and a big memory maker with kids.
also, Take your children for the walk out on the seven mile bridge, the old part. Yes, you can park the car at the parking lot that sits just to the right before the bridge. It is excellent, safe, and very Scenic. You can look down and see the big fish, sharks, rays, and the loggerhead sea turtles. We love the florida Keys Things to do with Children.
Seeing the Key Deer is very nice and the children can get close, if you do not spook them. In Key West, they have a Aquarium and marine life center, with shark feeding and things like that. They have sea turtles too, and you are so close that is like being at the reef.