LeslieG: You are automatically assigned one of the available seats when you check in. I have read that Airtran keeps the last 3 rows of seats aside to accommodate families, etc..... If you look at available seats on most flights, the last 3 rows are generally never available for "pre" selection.
In addition:
You know, for someone who has paid an additional $15 per seat, I would not be happy being asked to move to accommodate another adult

. My mid May airfare was purchased in mid January and my seats were purchased at that time. I wanted 3 seats together for my family at the front section of the plane, so I bought them. I would have no problem moving for a child to sit beside a parent, as long as I got moved to another $15 seat, or Airtran was willing to refund me my seat charge. Everyone has the opportunity to purchase their specified seat when they purchase airfare. If you want certain seats, or want to sit beside someone, buy the seats that will accommodate that need right from the "get go".
There are seats available for charges of $15, $13 and $6 each, it is not that much and $6 per seat should not put that much of a burden on anyone for the peace of mind knowing that they are going to be sitting where and with whom they want.
On our flight home (also purchased 3 $15 seats), I have noticed that there is only one set of 3 seats together left on the whole plane.
So families of 3 or more, who did not purchase seats, are going to be spread out.