I don't have children, but I come from a huge extended family (66 cousins, 32 second cousins, at last count), and I look after a toddler quite frequently, so I have a little bit of experience in this area.
First off, the airline was perfectly justified in what they did. The other passengers on that plane had places to be, some had connections to make. That airplane was needed down the line for other flights. They shouldn't be exepected to have to drop everything because some child wouldn't behave. Maybe they weren't "polite" enough (I don't know, I wasn't there.) But they looked after the best interest of the passengers.
Second, what's with these parents? If a child I was looking after had a tantrum like that, and I'm not fool enough to think it wouldn't happen, I would be mortified. I would accept the refund and free tickets gratefully, and I would keep my trap shut about it.
Note that I am not blaming the parents for the kid's tantrum, nor am I blaming the kid. Kids spaz out all the time, especially after what was probably a long day and a lot of excitement. But airlines don't have to be concerned with that: They're there to get people from point A to point B safely and efficiently, and as pleasently as possible. A screaming kid is not pleasent.
Bottom line: AirTran did the right thing (kicking them off) and the gracious thing (refunding and offering tickets). The parents are now the ones misbehaving. They seem like the type of people who - when they don't get their own way - make a big stink and accuse anyone who doesn't bend over for them of being "rude". There's lots of people out there like that (They're a dime a dozen in hotels and restaurants), and they're just big babies.
Just my .02