Beyond that, no Southwest flight would separate you from your kids. I have seen them ground a plane until someone lets a kid sit with their parent. And the boarding group that you'd be in even if you didn't do Early Bird check in (guarenteed A boarding group for an extra fee) always has plenty of seating.
I'm sure it's not a regular occurence, but it does happen that Southwest allows families to be separated. We flew back and forth from Rochester, NY to Baltimore, MD a couple of weeks ago (our first experience on Southwest). We checked in precisely at 24 hours both times, which put us toward the front of the B group on each flight, so we were able to sit together.
On the outbound flight, a family at the end of the C group with two children in the 8-10 range was split up throughout the plane. The parents asked nicely if anyone would move so they could each sit with a child. No one did, and the only assistance offered by the flight attendants was to identify the four remaining [scattered] empty seats for the family and ask them politely to hurry up and be seated. Granted, the children were not toddlers and didn't appear agitated about not sitting with their parents (perhaps that is the big distinction), but it still would have been nice if they could have been accommodated together.
That being said, we very much enjoyed our flight experience on Southwest. The boarding process (and corresponding need to either rearrange your schedule to check in at 24 hours, or else pay an amount roughly equivalent to a fifth ticket for Early Bird check-in) is annoying, and seemed to take far longer than conventional rear-to-front boarding, but the flight itself and the funny/sarcastic flight attendants were just the right combination of efficient and fun.
JetBlue remains my favorite, if only because I can pick our seats ahead of time, and enjoy a nice dose of HGTV or the History Channel as I fly. (The number of "free" checked bags is a non-issue in our analysis, as we travel "carryon only.") They're both good airlines, however, and I'm grateful that they fly out of both of our local airports.
It's always good to have choices.