Wow...never expected a simple question to cause quite a stir. But thanks for the info.
I just simply couldn't remember the policy, wasn't expecting anything that wasn't due to me or expecting anybody to give up anything they had paid for. I had actually forgotten that airtran had charged for seat assignments or I would have booked the cheaper less convenient Delta flight.
Dancing Dinsey- Yesterday when I checked out the seat assignment tool it plugged in $6 per person for all 3 flights-1 on the way down and 2 home. But when I just went and bought seats it only charged $6 for each way, not per flight, so you were correct, $48 total.
Don't worry about it. You sort of hit on a sore topic. I don't think anyone meant to offend you and am sorry if we did, but just wanted you to be prepared. If you do a search you will see that this question gets asks fairly frequently and some people always post ways to make sure you get to sit next to your child. They tell people not to worry about buying seats, that since you have a child Air Tran will make sure you get to sit next to that child, even if they have to move someone who paid for their seat. No True! Too many people on here have seen even very small children separated from their parents. Then some people come on here with ideas on how to make people move, (if you can board before them, just take the seat anyway and refuse to move when they get on, toss the kid's bag at them and tell them to take care of their child then, tell the child something like 'when you throw up, this nice lady will take care of you', tell the kid to cry and throw a fit, etc). Personally you ask me nicely and I may accommodate you (still refuse to move to the back of the plane though), but try a stunt like some people on here suggest and I'm not budging.
Twice I've had flights where I was on one of those smaller MD80 type planes that have two seats on the right and three on the left as you get on, where someone was sitting in my seat. In neither case was it a matter of sitting next to a child though. The first time a lady was in my row 9 aisle seat by herself. When I showed her my boarding pass and told her she was in my seat, she kept pointing to the back of the plane, but refused to get up. A FA came up and the lady pulled the no English routine. The FA finally got her boarding pass from her and told her she was in the third row from the back in a middle seat on the left side. Suddenly her English improved and she called me a b-tch.

The second time a couple was sitting in those seats with the boyfriend in the aisle seat. I told him he was in my seat and without even looking up at me, he just kept flipping through his magazine, jerked his thumb toward the back of the plane and said he knew it, but I could have his 32E seat, because he wanted to sit with his girlfriend. He didn't even ask, just told me what he was going to do. Uh no! Not happening. I was calm, but firm and told him to get out of my seat, that if I had wanted to sit in a middle seat in the back of the plane, then I'd have bought that $6.00 seat, instead of the $15.00 one I did. Come to find out, they hadn't even paid for their seats, just checked in at the 24 hour mark and couldn't get seats together. Again a FA came up and asked what was wrong. The guy finally stood up and tried to intimidate us with his size, demanding that she make me move to the back of the plane so he could sit with his girlfriend. Sorry wrong thing to do! This grandma doesn't get intimidated and neither do the flight crew. He was told his choices were to move to his assigned seat or get off the plane. As we were taxiing out, the girlfriend started giving me a hard time. I told her that since her boyfriend was in a middle seat, more than likely the people sitting next to him were not with anyone and I was sure one of them would probably be willing to trade their seat in the back for her nice window seat up front. She got angry and said she didn't want to sit in the back and it wouldn't hurt me to move back there, so her boyfriend could sit with her. I smiled and politely asked her if I needed to call the FA back to keep her from harassing me and I was sure it wasn't too late for the captain to pull back to the gate to have her escorted off. I then put my earphones on and went to sleep. These weren't young teens, they were like in their 40's.
Anyway, my long winded point is that because it happens fairly frequently, some of us that purchase our seats may get a little sensitive about constantly being asked to give up seats we have paid for, for people who chose not to and just assumed that everyone else would accommodate them.