A lap child is a baby who does not have his or her own seat, you would not be considered priority. On the flights I have been on, they have not called them first anyway maybe because mostl everyone flying to Orlando is part of a family. I think whether they call them or not is up to the discretion of the employees. Plus, you have assigned seats on Airtran, everyone does, so when you get on board doesn't really matter because you can't just sit in someone else's seat. Some people pay extra to choose their seats early, some people take a chance and wait to be assigned a seat by the computer (which does not take into account ages of the people in the party and will split up groups if necessary) at the 24 hour mark.
In my opinion, it's just not worth taking the chance of being split up just to save $12 or so bucks (if everyone else doesn't care who they sit next to, you can just pay for one adult and the child to sit together while everyone else takes their chances). Some people have luck in getting others to move but when others are paying for their seats, those people are less likely to switch.