Having lived in South Florida, I feel i must tell you that your friends have ambitious plans. I doubt their 1yo will remember any of it. If they really want to go to the keys, I would suggest they consider flying to Miami or Fort Lauderdale( a great vacation sopt for families,btw), which is on the Atlantic. Key Largo is only about 1-2hr from Miami and has a very nice state park (John Pennenkamp SP), good snorkling, good fishing. If they plan to stay in Key Largo, tell them to be sure they choose a hotel facing Florida Bay(west)--the sunset is outstanding and you can't see it if you're facing the Atlantic(east.) Driving from Key Largo to Key West takes about 4hrs. It's a beautiful drive, mostly two-lanes of 45mph bumper-to-bumper traffic. Pack a picnic. I will warn you--most of the beaches in the keys are not very good; because there are coral reefs off-shore, the beaches are littered with broken coral. Ouch!
Getting from Miami to the Gulf means driving across the Everglades to the other side of the state, exactly opposite of going to the keys. The Everglades are fascinating, especially in winter when the mosquitos aren't so bad. Summer is pretty miserable. The west side of the Florida peninsula is very nice. The Gulf of Mexico is much different from the Atlantic, calmer, greener, sand is very white. On the gulf, I would suggest Sanibel Island, or Captiva(fly into Fort Myers). Both are beautiful beaches well known for great shells. It is a very relaxing place to vacation and the Gulf seafood is some of the best in the world.
Hope this helps. Tell your friends to expect terrible traffic in south Florida, but it's a great place to vacation.
