I fly SWA several times monthly, and IME, the reality is that the gate agents will be fine with allowing a younger child with a B (no older than 12) to board with a parent who has an A, so you and the child could board in your slot, but your DH would have to wait.
Seat-saving in the rear of the aircraft is tolerated as long as you don't attempt to save more than 2 seats, and are saving for real people, not a "phantom" created to keep the seat next to you empty for the flight. Obviously, saving one seat is always likely to be more successful than trying to save 2. Don't try saving at all in the first 10 rows -- head straight for the back. Also, never try to use "stuff" to save the seats, the FA's hate that. If you need to save, sit in an aisle seat near the tail and defend the seats verbally, apologetically explaining that your husband and son (or partner, or whatever the relationship is) are boarding with the B group. Most people will move on. I don't recommend saving in the very last row, however, because if someone has gone all the way to the back row looking for a seat, they will have trouble going back toward the front to look again. Your best bet is about 5-6 rows from the rear.
Understand that THERE IS NEVER A RIGHT TO SAVE, SO IF YOU ARE CHALLENGED, YOU MUST GIVE UP THE SEATS WITHOUT ARGUMENT, because the FA will not rule in your favor.