I think Southwest's general policy for preboarding small children is under 4 yrs old, or else 4 and under. BUT..... On my 2 southwest flights to WDW, one of them didn't preboard small children at all (there were like 30 of those, lol). The other one only did preboard for 2 and under WITH A CARSEAT TO INSTALL. There were tons of small children for that flight, as well, and this cut the preboard line drastically. On the flight home both times, they did preboard but they reduced it to UNDER 3 (so through age 2). All of these allowed immediate family only, with the young child...no grandmas, friends, etc.
Oh, and on that one, there some people getting sent to the ends of their A B or C lines when they tried to sneak into the preboard line with "2-year-old" Kindergarten-age kids. That same flight, so many preboarders tried to save tons of extra seats for their other family members that the flight attendants escorted us ("A" Boarding Passes) to the seats we wanted, and made the families move their belongings.
So check in the 24 hrs early just in case. Definitely can't count on preboarding small children when most of the flight has small children with them!
Beth