I don't know if they do it in Pit, but SWA and AirTran always seem to have about the same fare from BWI to MCO, so money is not the deciding factor for me.
I have flown both SWA and AirTran, and I like them both. What always decides it for me is the fact that SWA does not charge fees to change your flight, whereas all other airlies do, so unless your schedule is set in stone at time of booking, SWA is a little better.
As far as the sitting together thing goes, don't let it get you paranoid. All of the planes you're going to fly for any airline from Pit to MCO are going to have rows of three, so your family is going to have to sit in two rows anyway. And as long as you check in early enough with SWA and all get group A boarding passes, you will all be able to get on the plane together and sit in two rows next to each other or across from each other.
Advice - and a plea - Since you will have a family of 6, PLEASE just go all the way to the rear of the plane as soon as you get aboard. The rear of the cabin fills up slowest, so you will have the best chance of getting two rows of 3 either across from each other or next to each other if you go to the back immediately. But this also means that your group of 6 with 4 kids, who will likely take a long time to get your carry-ons secured and get the kids buckled in and set up with books, toys, snacks, DVD players, or whatever you bring to keep them from getting bored, will not block the isle for the rest of the passengers getting on the plane after you.
I love sitting in the rear of the plane for late evening flights, because it seems to be the quietest part of the cabin and I can usually get some sleep. On flights that are not full, I also have a better chance of getting a row of seats to myself in the back of the cabin.
This will require only one small sacrafice - you will be the last to get off the plane when you land. But when you land it will likely take you a long time to get your carry ons out of the overhead bin, stow the kids' toys, books, snacks, etc, and get ready to deplane anyway, so it probably won't be much of a delay.