We have flown SWA at least 20 times since they started flying out of Philly in 2005.
Our kids were 5 & 8 then. We have never been separated but even before there was Early Bird options, we were careful to check in for our flight at T-24. Even on rare occasions that we've gotten B boarding passes, we still got to sit together.
If your child is 4 or under, you will be able to do Family Boarding which is after the A boarding but before B boarding.
Definitely give them a try. We have flown other airlines since and SWA has one of the smoothest boarding plans of them all. It's funny how others call it a cattle call but USAirways/American is the worst, despite their assigned seating, at how passengers just hover so close to the line that you can't tell if they are actually in line or just hovering. It's so annoying.
At least with SWA boarding, no one really gets close to the line unless they're in it. Once they started adding the number position to the letter designation, like A-35, it made it so easy. You just go up to the line and ask people what their number is so you can find your spot in line.
It's so civilized compared to American and not a cattle call at all.
We still buy EBCI for our return flights, especially when cruising, because I don't want to have to deal with checking in while on vacation.