I'm not sure what dates you were looking to stay at the St. Kitts Resort, but I found the following dates available using base standard awards with no upgrades (all 2009 dates):

Jan. 1-16, 19, 20, 23-29
Feb. 1-28
Mar. 5-31
Apr. 1-30
May 1-31
Jun 1-30
Jul 1-31
Aug 1-31
Sep 1-30
Oct 1-31
Nov 1-16 (end of loaded inventory)

So other than a few days in January and the beginning of March, every single day to November is available without any upgrades. To me, I can't expect a hotel to have better availability than this.

As far as the OP is concerned, I agree with you in that the Harbor Beach Club is about as good as it gets. It has a great beach location, great hotel and service, and great location to other activities in Ft. Lauderdale.

As for the Cayman Islands, if your boys like to snorkel or scuba, they shouldn't get bored with new places to explore. And they would probably love to hand feed the sting rays. The RC on the Caymans is on a great beach spot. (And even though the Marriott isn't on quite as nice a beach area, all you have to do is walk 10 minutes up the beach and your at the RC's beach area. ^