UPDATE: Just got back today and wanted to let y'all know how it went.
On the flight down (Monday, 2/18), I checked in at exactly 24 hours out via the SW website - with one hand on the refresh button and one eye on the exact time - and got our party of 5 boarding positions B4-8. There was much rejoicing... until we got to the airport and saw the hordes smugly lined up for family boarding (many of whom had children who looked clearly older than 6 or were gunning to have 3 or 4 adults board with one kid: despite the fact that both are against Southwest's published policy, nobody was checked or questioned). For a plane that seats 143, with boarding position B4, I ended up being the 127th person to board! (In other words, a whopping 66 people pre-boarded, or boarded with family/disabled boarding between the A and B groups.) The flight was full, but my party of 5 managed to score the very last row available -- which was, of course, literally the very last row. Everyone behind us, with positions B9 and later, was limited to single, center or aisle seats scattered throughout the aircraft - and the poor woman who boarded last got stuck wedged between a morbidly obese window seat passenger who was occupying 1.5 seats, and the aisle seat passenger, who had unhelpfully put down their armrest. She basically had to sit sideways for the entire flight! While I can't complain about where my family ended up (we still got to sit together, after all), I was genuinely surprised that our "good" boarding positions ended up being so terrible in practice, and that Southwest was showing so little interest in enforcing any of its family boarding restrictions.
On the way back (today, Friday, 2/22) -- for which I'd purchased Early Bird about 4 months ahead of time -- we got boarding positions A50-54; not too far ahead of our assignments without Early Bird, but on the right side of family boarding. This time, we were all able to obtain seats in row 11. I should add that those who purchased Early Bird much after the 4-month mark when I did would have ended up in the B group anyway, and would have been behind the several dozen family boarders who were sauntering onto the plane after the A group, waving their offspring around like golden tickets.
In short, I would warn anyone bound for Orlando (especially during a school vacation) that if you don't purchase Early Bird - and do so the second your travel dates become available - (and of course, if you're not otherwise entitled to earlier boarding or to family boarding) family boarding has the power to condemn you to some pretty undesirable seating choices if you're flying Southwest. I understand the reasons for the family boarding policy (nobody wants to be stuck sitting next to a 2-year-old whose parents are across the aircraft, and sorting everybody out would delay departures if any families with small kids ended up getting on last), but when you're stressing in the B group lineup, it is pretty maddening to watch dozens of people jumping ahead of you, especially when it looks like some of those people have no business doing so. For me, the no-assigned-seats policy is the fly in the ointment for an airline I otherwise really like.