Well, sometimes it depends on the flight crew and how many people are on the flight. Twice before (the most recent being Oct. '04), we were allowed to preboard even though our DS was over 5 yo. On our Oct. flight, there were prob. a total of 4 families so they allowed all parents w/children to preboard. Now, the flight wasn't crowded, so I'm sure that this had a lot to do w/it. So, regardless of A, B or C line tickets, they pulled the families out of the lines and allowed us to preboard. It was a nice treat and not one person complained. Again ... there weren't a lot of families or passengers.
However ... on the way home, we ended up in the C-line and the crew did not pull any families (other than those w/kids under 5 yo) out. We weren't able to sit w/our child. DW sat behind DS while I sat in the row across from him. We were fortunate b/c a teenage girl moved one seat over so DS could be closer to me! That was very nice of her and although we thanked her profusely then, we still appreciate her kind gesture today! Now, I am not complaining. It was only fair that we waited our turn b/c that is how things work. We weren't asking nor looking for special treatment b/c of the fact that we got it the last time around. We got what we deserved ... and we knew that we might not sit together when we found out we were in the C line. We knew it was through no other fault than our own for not getting to the airport earlier than we did. It was a chance we took and we accepted the consequences. If we had known the WDResort could print boarding passes, we would have been able to sit as a family, I'm sure.
As for people moving to accommodate others ...
Anyway ... the main point is that you never know what SWA will do as far as seating goes! If your flight crowd is light, they might allow all families to preboard. Or, you might get stuck in your line. It's a crap shoot.