Ribbot, I think the poster was recalling an incident that happened last year, that was before you could pick seats on Airtran. Believe me, I fly a lot for work, and I see families split apart all the time, even on carriers that assign seats (USAIR, United, etc) Depending on your fare and when you book, and how full the flight is when you book, even with those airlines you don't always get your seat when you buy your ticket. Sometimes your seat is assigned at the airport.
The airlines are in no obligation to ensure that your family stays together. If it is that important, I'd pay the 5 bucks. If it isn't that important to you, then take the gamble. It is not a crock, I see it all the time, just not sure how the recent change will work with Airtran, since now many of the seats are being spoken for, and charged for. And when families are split up, if you ask the attendant, they may try to get people to move around. But if people paid extra for those seats, the chances are they are not going to. Other destinations probably would not be affected as much, but the majority of people going to MCO are families (and not as many business travelers, other destinations it is the reverse) who do not want to be split up either.
That being said, I think if you check in 24 hours ahead of time, I think you have a very, very good chance of getting three together. I just know for me, when things can go wrong, they usually do, so I try not to leave too much to chance!