DD and I flew Airtran in November from Atlanta.
When I bought the tickets, I paid for the seats for us. There were very few 2 seats left together and DD doesn't like to sit next to strangers. I bought them a month before our flight and most of the seats left were singles.
When you check in on line you can chose your seats but if you have a small child that needs to sit with a parent, I'd pay for the seats upfront.