Like Bozzly I've used that tactic to pay for the cheapest car and know that the chances are I'll get a free upgrade, it's always worked for me BUT it's not a tactic I would use if I knew I couldn't make do with the smallest size car they offer , in the unlikely event the blighters actually have one and give it to me.
Wendye I think you're going to struggle in a full size not only seating but more importantly luggage space. I would consider a 7 seater minivan in your situation. The extra space when touring around is going to greatly increase everyone's comfort levels. IMHO there is little as nasty as getting into a REALLY HOT car at the end of a hot ( and sweaty day) in the parks to squeeze up next to a hot and sweaty person and drive 30-40 (or more if going to Tampa) minutes. The A/C may cool the car down eventually, but for those in the back seats the parts of their bodies that are pressed up against the others will remain hot and damp for the entire journey (I've made myself feel quite queezey just thinking about it ROFLMAO

) . It will be MUCH cheaper to order the bigger car here than wait and upgrade at the airport.
Suzycute, the aspect for you is, IMHO, not seating, but luggage space, as a father of two teenage daughters who did a little ( UNDERSTATEMENT!!!) shopping their last trip I know that by the time they were ready to go home they filled up my boot ( trunk) space on their own. If your family travel light, then you may be ok, but if everyone has their own suitcase, you're going to find it difficult. One thing that may help you to deal with luggage space is to make sure your family make use of soft luggage. Obviously you're going to need some hard suitcases, but if you can get by with two hard cases and use sportsbags type soft luggage that can fit in and around the hard cases you'll be able to deal with a much smaller trunk. Again it's much easier to fit 2 or 3 sports bags into the back seat spaces than it is to fit one hard/solid suitcase. It's only going to be getting from and most likely TO the airport that's going to be the issue, but leaving a suitcase (or daughter) behind is not going to be an option so selecting bag/luggage types with boot/trunk space in mind is going to make life a lot easier for you.