Worst flight ever would be one that crashes into a fireball. Second worst flight ever would be one that lands on a levee in Louisiana or the Hudson River.
Then there are horrid flights (3rd on down)... diverted to a remote airport, diverted to an airport in a country where you have no entry visa, lavs overflowing, sexual assault in flight, etc.
A tarmac delay and screaming kids is simply a bad flight. This happens on EVERY airline.
The FAs should always give people water during a tarmac delay. Other than that, there is nothing else Southwest could have done.