Way back in the dark ages (February 2002) - Southwest hadn't gone over to the A-B-C boarding passes yet. Everyone got a boarding pass with a number 1-130? on it, based on their check in time.
We lost some time on the 2 hour trip to the airport, and ended up arriving only about an hour and 15 minutes ahead of time, to find a HUGE line at check in (No self service kiosks back then, no online check-in, and no curbside check in at this airport). By the time we checked in, then went to the gate to pick up our boarding passes, our family of five had gotten #86-91. Back then the aisles were marked 1-30, 31-60, 61-90 and 91+. We sat right down in the 61-90 queue, and waited for our flight. Before the check in - they said to check your boarding pass, and be sure you were in the right queue - sure enough - guess who's pass had number 91 - my just turned 5 year old. There was NOO way I would let her go in that line - since you couldn't even see it from where we were - so I sent DH to go line up - she was crying "I can't get on the plane without my Daddy!" We were the first in the 3rd group to go through the gate (I was just going to save a seat for him). When the gate agent saw our passes, and I explained the situation - He made a special call for "Daddy", and ushered him onto the plane ahead of all our group. He said "No little girl can fly to Disney without her Daddy!" No one seemed to mind! (By the way - it was Daytona Bike week - our plane was filled with bikers heading down - not Disney family travelers!)
We were able to get a full row, as well as two seats right in front for our family to stay together, even though the plane was overbooked! So - even with a B, or possibly a C, pass - you should still be able to get seats together - just not in the front of the plane! If you do get unlucky, and get a B or C, just head straight to the gate so you can sit on the floor in the line, and be the first in your group!