Just thought I'd update what happened... Dh went to the service desk before boarding started and asked if we could all board with him so that we could be seated with the children, and he was told no. (She did say he could try to "save" seats for the rest of us.) After group A, about 6-8 more families boarded, most with school aged kids like us. (I was under the impression the family "pre board" was for kids 4 and under.)
There were maybe 30 people left lined up for B group. The attendant who was in charge of boarding then went down the line and pulled all the remaining families out and let us board before starting the rest of B. There were only 2 or 3 families who weren't trying to "pre board", so I was feeling cranky for being a law-abiding citizen and waiting, but was really happy when the attendant pulled us. When we got on, there really weren't many "multiple" spaces left, including all the way to the back. A nice guy about mid-plane was alone in an aisle seat, and offered to move up to another row so we could have a three. My dh got the aisle seat behind us, with another family. Worked out ok, but only through the kindness of strangers.
All that aside, we had a great flight. I heart Southwest. We were on time coming and going, the staff was pleasant and efficient, and none of my bags got lost. The parents of the two boys in row 14, however, need to set some boundaries for those children! I've never been on a noisier flight. I have NO ill feelings for the 2 mo old boy and his mom who was doing her very best--his ears hurt, or his teeth, or something, and he made a lot of unhappy noise, but Mom really did her best--it was the parents whose kids were old enough to know better, but didn't.
Thanks for all the feedback. Hope my experience is useful information for someone.