I fly Air Tran frequently. The only time I've ever seen preboarding was once for a Make a Wish family. I have seen people with kids ask and every single time, I've heard them told that Air Tran does not do preboarding except for Make a Wish kids and wheelchairs. Air Tran uses a Rotating Zone method to board. They start with business class and any wheelchairs first. After that, they load zone 1 which are people that paid for those $15.00-$13.00 seats in rows 12-16. They then start loading the last few rows, then move back toward the front, then the middle(usually starting with window, then middle, then aisle seats in each zone--though that seems to change with each trip. Sometimes sitting in the same row we have different boarding zones and sometimes we have the same). I know once we bought $6.00 seats just past the wing and even though we were in the same row, we had three different zone numbers. They always make an announcement that if your boarding party has different zone numbers, you may board with the lowest number though, so it's not a big deal. My husband and I boarded with my mom, who had a window seat and had a lower zone than either of us. Zone 7 is usually last and are those people in the first few rows of coach that did not pay for those $15.00-$13.00 seats, but instead picked them for free at the 24 hour mark. This past Dec I was sitting talking with a lady in the boarding area and we noticed that we were sitting next to each other on the plane in row 12. I had boarding zone 1 and she had boarding zone 7. Out of curiosity she went up and asked the desk agent why. After looking at both tickets she immediately said that I had apparently paid for one of those $15.00 seats and the other lady hadn't. Sure enough, the other lady chose her seat at the 24 hour mark for free, whereas I had paid for mine months before. The desk agent knew immediately though, just by looking at the seat numbers and the boarding zone we had been assigned.
OT: If you pay for one of those $15.00-$13.00 seats close to the front of coach, it says when you click on it at the time of purchase that you are guaranteed Boarding Zone 1. If you pick that same seat for free at the 24 hour mark you will be in the very last boarding zone (normally zone 7). The site doesn't warn you about that though.
Don't worry about the stroller and car seat. Flights to MCO are always full of families with kids. You'll just drop the stroller off right there before you get to the door of the plane and there will be others with car seats. No one is in that big of a hurry, plus they usually wait a several minutes between calling each zone, so there shouldn't be that many people behind you waiting anyway. Take your time and get settled.