[aaomidi]

Also it means if you're a family, you're going to get split apart.

However, it does mean that boarding is done significantly faster.

[hnburnsy]

Families with young children board together 1/3 of the way through the boarding process on Southwest. They will be seated together.

[robotcapital]

Though that does presume being ready to board at precisely that moment while traveling with small children. In my experience I can expect that to happen about half the time.

[secabeen]

If you are clearly a family, and miss family boarding, I believe they'll let you board whenever you're ready. As long as you get on sometime in the "B" group, you should find an empty row or two at the back of the plane.