I've only recently flown Southwest with my children (June) to Orlando but they offered family pre-boarding.
We left Pittsburgh, but had A passes so we boarded with those. Coming home they offered family pre-boarding, too, and we had mid-B passes so we boarded between A and B.
I wonder if it changes things that the flight from PIT-MCO was the first leg of it's journey? Not everyone on our plane got off in Orlando, the flight had three more stops actually.
Oh, I'm also fairly certain they did allow entire families to board as part of the family pre-boarding. I admit I wasn't paying close attention, though, as it was just my husband and I with our children who were both eligible. So even if they'd denied entire family, we'd of been able to board together.