If Disney hasn't indicated already, make sure they gave seat assignments (if Air Tran assigns them, they may not, I can't remember). If Air Tran does permit advance seats, make sure you're together. Double check you seats about 1 month prior to travel, then one week prior. Sometimes the airline will change your seat assignments.
Some airlines require that you have a frequent flyer number to be able to check in online, I'm not sure if Air Tran is one of them. If they are, and you don't have one, you can check in at the airport.
As for tips for traveling with young kids, the best thing to do it to allow PLENTY of time at the airport. You'll be moving much slower with kids and luggage, it's better to be early and sit and wait, than try to run for your flight with little ones in tow!
If you are checking bags, DO NOT put ANYTHING you need for the first 24 hours of your trip in your checked bags (especially medication). Also don't check anything you ever want to see again (cameras, for example). If they lose your bag, you may not see it for a while. I recomend you carry on the plane at least 1 change of clothes per person, swimsuits, esential toiletries, medications, and cameras/other valuables. Even with 4 people you should get that into one small suitcase.
If you are traveling with a car seat and/or stroller, you must take the child out, fold up the stroller, and walk with the child through the metal detector. If the seat/stroller can fit, they must be x-rayed. If they can't fit through the machine, they are hand inspected. I recomend that you send one adult through first, then the kids, with the other adult bringing up the rear. If one of the kids sets of the detector, they will be wanded just like adults, so make sure your kids don't have outfits with a lot of metal snaps (like overalls). You can take your stroller through security to use until flight time, then gate-check it. They'll tag it and take it below to be put in cargo, and have it out for you when you deboard at your destination, ready to use.
The airlines do not have meals on flights anymore, although you can sometimes buy snacks. You can (and should!) bring your own snacks along for A) delays B) to cover a missed meal time C) keep 'em busy and quiet. Also, especially for your two year old, brink a sucker, a bottle, or a sippy cup for them for take off and landing, the "sucking" will help him/her pop their ears, making them much more comfortable. You can do the same for the five year old, or tell him/her to yawn frequently to relieve the presure. Also, brink small toys or travel games (something that is easy to carry but doesn't have parts that could get lost) so they have something for the flight to keep them busy. Try and make it either a favorite or something new to keep their interest.
A good packing tip for kids is to take the large gallon sized zip lock bags and put one days out fit for each kid in a bag. Top, bottom, undies, and socks. Makes it easy in the morning to grab a bag and help get them dressed (or let them pick their own outfit). Then you can put the dirty clothes back in the bag at the end of the day, keeping dirty separate from clean (really great if the clothes are WET).
