I'm not aware of any seating charts for SW planes. All of their planes are 737's so there are 3 seats on each side. (I believe one side of one extra row may have only 2 seats, but you won't be able to sit there since you have children.)
Honestly, I'm not sure that it matters where you sit. If you want to be able to see without the wing in the way, I'd recommend avoiding around row 12 through 18. Obviously, the front is the most popular, since you're off the plane quicker when you land. If you're concerned about sitting together, you may want to head to the back. But, frankly, I'd just grab the first 4 seats you come to that are together.... three on one side, and one across the aisle.