Miami Metro Zoo is great. There is a new Children's Museum along with Parrot Jungle on the causeway to South Beach. There is also the Venitian Pool in Coral Gables. Kids should enjoy swimming there.
Miami Seaquarum is a small version of Sea World with interesting shows.

Along the way to Key West, you can feed the Tarpon's at Robbie's in Islamorada. Mile Marker 77.5 - Robbie’s Marina/Restaurant - Funky little collection of things. Waterfront dining, feed large tarpon, kayak to nearby islands, little shops. Worth a stop. http://www.robbies.com/tarpon.htm

Eat at Blue Heaven in Key West for breakfast or lunch. Outdoor dining, music, wild chickens to entertain the kids.