I fly Southwest about once a year, and they let all families with children board between groups A and B.... at that point, you shouldn't have any problems getting seats together.
Also, with Southwest, you "check in" online 24 hours before the flight time, and that is when you are given your "boarding order"....A group is first, of course, then B, then C.... and the lower your number, the better. I believe there are 60 numbers in a group. I usually am able to get about A 30 by doing the online check-in right when it opens up 24 hours before the flight. My last trip, I forgot to check-in online, and I don't remember what number I got (B something, I think), but still was able to find the window seat, which is what I want.
As someone else said, though, most times people are willing to trade to keep kids and parents together.