You are given a boarding number when you check in. If you pay for early checking, SWA will do this for you at the 24 hour mark & alert you to your boarding number. If you do not, you want to checkin 24 hours prior to your flight (EXACTLY) to get the best seating order. If you booked the family together, you will get consecutive numbers so sitting together is not a problem. If you made separate bookings, you will want to be on several devices to check everyone in. Sometimes I book DH & I separate (maybe I have points and I have to pay for him with cash). I will have one typed in on my phone, and another on my iPad or laptop a few minutes prior to the checkin time. When the 24 hour prior minute clicks, I click checkin on both...we are never more than 5 spots apart. Whoever has the better spot usually just hangs back with whoever has the not as good spot.
If you checkin EXACTLY 24 hours early you will usually get pretty decent seat selection.