WOW!!!! I've never seen such a group of nervous nellies!
Here's MHO:
I fly SWA 20 - 25 times each year. ONCE, in the last 5 years I witnessed a family rush onto a full plane at the LAST POSSIBLE SECOND

(everyone else was seated and they were about to close the door and push back) and then act surprised

that they did not see 4 seats all together saved especially for them. The nice flight attendant

found a gentleman willing to switch his seat so that the LM and CS could sit together. Dad and older daughter had to sit separately several rows apart. ( Keep in mind that although this was probably their fault, there was a slight chance that their connecting flight, on another airline of course

, arrived late.)
SO, if you cannot print your boarding pass just after midnite, arriving at the airport 2 hours before your flight will most likely get you in the "A" or "B" group. In my experience, you don't have to worry AT ALL if you get an A or B. Even with a "C" chances are that you can find 2 seats together. If you're travelling with a child under 5 they will let you preboard and at least one parent will sit next to their child - even on a packed full flight.
Keep in mind that current SWA load factors are about 76% so their are many flights leaving with some empty seats. Instead of staying up until midnite, rushing to the airport at 3am and worrying -
relax and think about the wonderful time you'll have at WDW.
SWA is #1 and remember what I overheard the pilot telling the little nervous boy who said he was going to Orlando and asked the pilot where he was going. The pilot replied,
"I'm going to Orlando and so is everyone else on this plane." The little boy (now smiling and happy) turned to his parents and said, "Everybody on this plane is going to the same place!"
*LM=Late Mother, CS=Crying Son.