You can't really "adjust" to the humidity in a span of one week, or even two weeks. All that you can hope to do is to deal with it in a healthy way.
Everyone has given you great advice. A mid-day break would be my biggest piece of advice. Arrive at the parks early -- leave at lunchtime or so, swim, have a nap ---- return in the evening.
Also, in Florida, the sun itself is brutal. Please, please, put a brimmed hat on your little one, and use plenty of sunscreen. Actually, everyone in your party should wear a hat and sunscreen -- re-applied several times during the day.
One of those water-mister fans might be good to keep in the stroller. Oh, yeah, even if (s)he doesn't still use a stroller, you will probably want one in the parks.
I have lived in Georgia all my life. The humidity is very high in GA --- but it's higher in FLA. You really have to be aware and be careful.
Above all, have fun!