A child under 5 may qualify your entire family (should you not get A group passes) for boarding between the A and B groups (SWA no longer allows families with young children to board prior to the A group) or, if the gate agents decide there are too many family boarders, may qualify only the eligible child and one adult to board between the A and B groups.
Unless you have a high B or a C pass or arrive at the departure gate after most of the other passengers have already boarded, the odds are good that a parent will be seated with the children. If you do not have an A pass the seats together will likely be toward the rear of the plane.
The absolute best way to ensure that your entire family boards first is to book Business Select tickets. Holders of these tickets are guaranteed a low-number A group pass. But they are expensive. The second best way is to check in as early as possible to try and get A-group passes for everyone. If you use family boarding, even if you don't get A passes you will be okay as long as you don't mind the back of the plane. If you feel sitting in the front of the plane is an absolute must, you need to check in online or with a web-enabled cell phone exactly 24 hours from your flight time.