We only fly Southwest when we go to Florida, it's so much more pleasant, and the flight is non-stop. There are five of us.
Just an FYI, southwest planes only seat 3 in a row, so you will probably split up and sit one child with one adult, either next to or right behind one another. We always split the older two girls 11, 14 next to one another, and Mom. Dad and Sarah (5) right behind them. We have never had a problem with this.
Last March, we were delayed getting through security and when we got to the gate (This was before you could get boarding passes at checkin, when they were numbered), we got numbers 86-91. We lined up at the gate in the 60-90 chute right away, so we would hopefully get seats. We sent Dad over to the 91-end line, way across the room, and told him we would try to save him a seat. It turned out my 5 year old was supposed to be 91, but there was no way she would wait by herself. When we got to board, Sarah was crying that she couldn't get on the plane without her Daddy, and the boarding agent called John over (recognized him by his red face

) and told him to get on the plane! We still had no trouble getting two rows together, although the plane was overbooked, and someone did end up sitting next to my big girls.
If you check in at least an 1 1/2 or two hours before your flight, you should have no problem getting an A or maybe a B pass, and will have no problem getting seats.