There are no assigned seats on SW. You get a boarding number (A1-60, B1-60 or C1-60). We buy the early check-in when we purchase our tickets which has SW check you in 36 hours ahead of the flight instead of 24. There is usually family boarding between A and B. You could purchase the early check-in for a few of the adults and then just save seats for the few others. Otherwise be on the computer just before the 24 hour check-in and get checked in as soon as possible.