Hi, Kim ...
Karen's advice is excellent. I'd go with the blue blazer for your son, if he likes to dress up. I'm sure there will be "events" -- weddings, funerals, maybe even a cotillion -- before he grows out of it.
If you have trouble with a dress for your daughter, a black velvet or other fabric skirt with a dressy blouse and a neat belt could be a solution. She could also wear black silky pants with an interesting top. This makes fitting a LOT easier.
I don't know of any 12 YO girls who (a) take fashion advice from Mom or (b) would be caught dead in what we used to call "party dresses."
I'm not so sure about the trolley/train deal in Key West. I think they both make all the stops, but I'm not certain. Check to make sure. Get your tickets after you get off the ship, saving about $3/pp. The waterfront area is fun to walk around, the beach at the Flagler got good reviews on our last trip, the Hemingway house (and the associated cats -- 60 or so of them) is a neat stop and a stroll down Duval street is a must.
On Coz, since you are four, I'd just get off the ship, rent a car right at the pier (plan on $50) and go exploring. Plenty of English is spoken on Coz, so that's not a worry. The drivers are reasonably polite. My favorite beach these days is the Palancar Beach Club. Turn right out of the airport and keep driving past Chankanaab and Playa San Fancisco. You can then circle the island (doesn't take too long) with possibly a stop at Playa Bonita. Don't bother with the ruins at San Gervasio. You can top up your gas tank as you come into San Miguel. Then park and enjoy the town. You have plenty of time for all this.
Another option is to jump on the ferry at the downtown dock, cross over to Playa del Carmen, then go to Xcaret for the day. Kids love it. You can catch a cab to Xcaret from Playa and there are plenty of cabs waiting to take you back. The ferry is $8/pp, a cab should be less than $10 for all of you, and park entry is around $35/pp.
As to your cocktail pants, I can't remember the last time I wore a dress on a ship.
If there's anything else we can help with, just ask. I did this itinerary twice last year (not on Fascination) so my recollections are still pretty fresh.
pamda -- CM First-Time Cruisers Editor.