SW does not have seat selections. They will give you a boarding pass, either A, B, or C when you check in at curbside or check your luggage. They have you board the plane by your pass and you pick your own seats. If you have a child under 5 they let you board the plane (with the other families with under 5 kids) before everyone else is seated.
I wouldn't be too concerned about being able to sit together. I know on our flight last year everyone seemed to make out just fine. I have heard that they will ask people to change seats so you can be together.