A few things you should know about AirTran. I personally like them. Remember coach is pretty much the same on any airline.
How can I reserve my seat before the flight?
Customers traveling in Business Class or on full coach fares (Y or B class) may request a seat assignment at the time of booking. (The reservation agent can advise you of the fare class you have secured.) When booking these fares online, look for the "Reserve Seats" button on the Receipt page at the completion of booking. Seats on more deeply discounted fares, such as "Q","L" and "H," are assigned upon check-in for your flight.
If you have to have assigned seats months in advance AirTran is not the airline for you. Arriving early doesn't guarantee a family of 4 to be seated together because elites and full fare patrons will be accomodated before you. A small child will not be separated from an adult but there may only be one adult with a child. For example Adult A may be seated with Child B in Row 10 and Adult C and Child D may be seated together in Row 26. If this isn't acceptable book with another carrier.