Where to go for fun in the sun?

My family would like to escape the cold for a few days in March. We have a 3 month old baby and a 3.5 year old daughter. We'd like to go somewhere warm for about 5 days. Does anyone have suggestions on where we could go that would have some fun things for our 3 year old? We don't want to do Disney World yet. The only stipulation is that I don't want to be too far away from decent hospitals. Cruises are definitely out. The baby has a medical condition and if she were to have a problem while we are traveling, I want to be relatively close to a decent hospital.

There's good information on http://www.wejustgotback.com/ for family travel ideas.

Also heard great things about this resort in Jamaica that has a nanny designated to each family. Friends of ours went recently and really had a good time.
http://www.fdrholidays.com/fdr/index.php