Don't know about the parking, I kind of doubt there is a free lot and you can't wait outside for security reasons. It takes about 30 min to get to MCO from WDW, maybe less than that at night. I would wait for their call at your hotel and then go. By the time they get their luggage (if they are checking luggage) you should be there.
Air Tran is on A side when you enter the terminal, follow the signs, then look for the arrivals curb which should be right outside baggage claim. If they wait there, you should be able to pull right up and pick them up.
Edit - Fran has a good idea if you are staying at a WDW resort, let ME transport them.