A lot of people here fly Southwest when going to WDW, so I thought I would share this article from TPG about why purchasing Early Boarding rarely makes sense for young families:
Also, here is an article on Southwest’s EarlyBird Check-In that includes some situations where it may be worth it:
We did buy early boarding our last 2 trips and our youngest fits the family boarding category. For one, I didn’t have to remember to check in at 24 hours, especially for the return flight when we were in the parks. Also, there are a lot of people in family boarding heading to and from Orlando from our home airport. On flight in last trip, there were as many people boarding in family boarding as there were in the A group.
But these would be good tips for flying somewhere other than Orlando for us. 
From the author:
Since my kids are two and five years old, still within the Family Boarding age range, I have boarded with Family Boarding on every single flight.
So yeah, it’s probably not worth it for him…and probably not for another four years.
But for those of us with kids older than 6, then it’s worth it. I don’t want to have to add “Check into Southwest” into my touring plan; last year on a trip to Vegas, I was 10 minutes late to check in and we got “C” boarding. Fortunately, my wife and I found two seats together, but we were prepared to have to fly separately.
If my kids were teenagers, I’d probably think differently. But with kids aged 6-12, I automatically factor in EarlyBird when comparing airfares and flights if Southwest has the best flight.
I actually attempt to sit separate from my family on all flights! Solo air travel is so much better. I got some looks and there were some things muttered that I didn’t quite hear one time when coming back from Houston and DW shared a row with DD6, DS4 and DS9mo (on lap). I’m not sure what the problem was, I just put my headphones in and watched a movie.