I'm sorry that happened to you, but can I request that you PLEASE post this story more frequently? there are people here who insist that this will never happen to them and we are just inventing things to upset parents.
Posts like the one above yours do no service; actual fact and reality are what is needed here, and it is sorely lacking these days.
I guarantee the shortness that was displayed was out of frustration from the gate agents who hear it over and over that THEY NEED TO SEAT the family together. The response is basically saying you should have paid for seats.... albeit it could have been said nicer.
The fact out there is more and more people are being proactive and paying for seating. Those that choose to not to pay have to accept that they could be seated apart.
Now, in the case of the OP, no seats are together. It doesn't hurt to call Airtran and explain the situation. They do block out seats together. If they can't help, get to the airport VERY EARLY! They typically start accepting luggage for flights 4 hours in advance. Get there then and ask to see if you can be seated together. Be proactive.
I have told this story before, but here it is. We were on the same flight as some friends (MSP-MCO). We booked our seats ahead of time but they did not. They had booked through Disney and at that time had to wait until airport check in for seating. I told them to get to the airport early. In fact, we had assigned seats but wanted exit rows (they were only assigned at the airport then). We got there at the 4 hour mark, got our exit row seats and headed to the Mall of America for some fun. The seats we had originally booked were now released. There were open seats together! Our friends didn't take my advice and they showed up at the airport about 1.5 hours before the flight. Guess what, there were no seats together. They asked again at the gate and still could not be seated together. They had to ask on the plane for people to move around so they could be seated with their kids. They ended up getting 1A with 1C together in two different rows. Now, this was before PAYING for seating, so people were more willing to work with them. Now that people pay for their seating, the game has changed.
The moral of the story, be proactive. Pay for your seats (if you can) or be ready to deal with whatever happens. Choosing not to pay and then spazing at the airport will do no good.
Best of luck OP. I know your situation is a bit different than my scenerio, I hope it all works out.
Duds