Are you regular campers with the normal camping gear?
Sleeping bags are rated for the temperature they are good down to. Sheets and blankets will feel much colder than a sleeping bag which completely surrounds you (or you and your SO if you zip 2 together) . A "normal" sleeping bag will be fine down into the upper 50's but check yours. Colder than that and you'll probably want one rated for colder temps, or plan on layering extra blankets on top of the sleeping bag.
An air mattress will usually make you feel warmer than sleeping on the ground. From a thermal dynamics perspective "trapping" cold is not possible, and as far as conducting heat is concerned, confined air is a pretty good insulator. But if you just have a sheet between you and an air mattress and it's chilly it's true you will be cold.
Other tips:
Keep your heads covered at night. The low-temp mummy sleeping bags will have a tight hood for your head, but you can gain most of that benefit by having a stocking cap available to wear to sleep if it's particularly cold.
Stay dry. If you are too bundled up and get sweaty that will cool and eventually freeze (well, maybe not in FL) once you become less active. Layers are a good thing
Easy on/off shoes that are warmer than sandals. If you're getting up at 3AM to walk to the bathroom, you may want something with more coverage than a sandal but quicker to put on than tennis shoes.
Leave the windows open a bit. While it seems counter-intuitive, if you close up your tent tight people breathing will drive up the humidity to near 100% and you'll have condensation forming making everyone damp. If your tent has permanent ventilation areas you may not need to do this.
Electric heater? Not sure if they have policies against using them, but a small electric heater should keep the tent several degrees warmer, which in the mild Florida winters should be enough. Make sure to have adequate space around it, and choose a model with the more current saftey interlocks (tip, over-temp, etc.)
I've camped over ~150 days when it was below freezing, and probably 20 days below 0. Its a different experience, but in a Disney campground where you don't need to pack in your gear, have hot showers available, and the average daily temp should be in the 70's it shouldn't be much of a hardship.