... the flight has a lot of children booked on it. At a lot of SWA locations, it has to do with the way that the gate area is set up. It happens a lot @ Midway in Chicago, where the preboard areas are in stairwells. There isn't room for more than about 5 people to wait in the preboard area, as it blocks the de-planing passengers' egress from the jetway. IME, at Midway they will only let you preboard if you are disabled, or if you are securing a carseat. Usually that is regardless of destination.
When they do restrict, most commonly, they will restrict to one adult boarding with one <5 child. However, since most adults travelling without children do not wish to sit next to an unsupervised child, most people turn a blind eye to judicious seat-saving. If you and your younger child are the only ones allowed to preboard, sit on the 2-side, and put yourself and your child in aisle seats, one behind the other, then sort of hover in front of your seat (but clear of the aisle.) When someone asks if the seat is taken, smile and say that that you are waiting for the rest of the children; very few people will argue. When your DH boards with your other child, put one child in each row. If it is a crowded flight, try to take the center of two rows on the 3-across side; individuals will take the aisle seats, leaving the window seats open for your DH and your other child.
Remember that to most adults, and especially business travelers, the aisle seats are the most valuable. They generally do not like the rear-facing seats (no recline), but those are actually good for family parties, so if you take the rear-facing seats with children, you will usually scare other adults away from the front-facing seats that face into them. (Those front-facing seats have really good legroom *if* the rear-facing seats stay empty; if the rear-facing seats are occupied, the legroom is nasty.) One caveat to those rear-facing seats; there are no tray tables.