How old are your kids??? SW allows the immediate family traveling with a child under 4 to board inbetween the A group and the B goup. But if that isn't your situation, I would be tempted to pay for EBCI. Yes, it will add $40 to each leg, but it may be worth it to not have to worry.
You pay the $10 pp, each way. Then, SW checks you in 36 hrs before flight departure. You don't have to worry about doing it yourself at the 24 hr window. Those who have paid for EBCI get lower boarding numbers than those who wait for the 24 hr window.
As far as trying to leave the middle seat open...you can try, but it may not work. SW if flying more and more flights almost completely full. I was on one SW flight that had a largish family, maybe about 9 or 10 of them, on it. They boarded and then spread out over the first 2 or 3 rows on both sides of the plane. They took the aisle and window seats and put a belonging in that middle seat. It was this way from about row 4 back to row 6. Well, people came along, looked, and kept looking for seats together. After the B group had boarded, the FA came over the PA and announced that this flight was 100% full...there were no empty seats. You should have seen this family scrambling to get seats together rather than have a stranger in between some of them. All of a sudden 2 rows emptied and were now available.
Their faces were pretty funny.
I would look for an empty row and have a child sit on the window, then a parent in the middle, another child on the aisle and the other parent across the aisle in the aisle seat.