I flew once with two kids that just missed the cutoff (think it was four and under). We arrived really early for the first flight and waited forever in the line, but received first group to load. No problem, right?
WRONG!!
We had to connect in Chicago and were late arriving. By the time we got to the gate, most of the crowd was already boarded. We had to walk up and down the aisle and ask about ten people if they could swap seats before we finally found two sets of two. It was getting to the point that I was going to fly back home, because I would not have my five year old son sitting by himself next to strangers.
That was the last trip we took on Southwest. Ever since, I've flown mostly Delta, with a few AA flights in between.
This past May, I was able to arrange a trip to MCO for 17 people out of KC International on Delta Express. When I checked the fares, it was $210 each on Delta Express and $259 each on Southwest. So, check around. The other airlines usually match or come real close to Southwest without the seating hassle.