To gfolchick,
The following recommendations assume relatively normal numbers of passengers. In December, travel may still be below normal due to the aftereffects of 9/11.
Southwest preboards kids at most airports, but not always at Orlando, as many of the passengers have little kids. The last time I flew SW (leaving Orlando), they only preboarded families with kids under 2 years. You could always ask at the gate upon arrival in Orlando as to what they are doing in December. I don't think you'll have any problems preboarding on your initial flight TO Orlando.
Regarding seats, SW planes have regular seat--3 across on either side of a single aisle. In addition, as posted earlier, some of the planes have lounge seats which face each other--One row has been turned around to face backwards. It you can get two seats facing forward and two backward (facing each other), this might work well for your group. THe kids get to face the parents and the parents get a little extra room. It also feels less crowded as you don't have a seatback right in front of you. Also, if the plane is not full, there is a chance that the other seats in the rows may not get taken, which would give you even more space.
Regarding boarding, SW starts handing out boarding passes 1 hour before departure. Passenger often line up earlier, so as to be near the front of the line, The boarding passes are simple plastic cards with numbers (1 thru however many they need). In the past, one person could get all of the boarding passes for their group--I don't know what happens now, since 9/11--maybe some traveller can help us out.
Anyway, there will be groups called to board the plane at the appropriate time. IF you can preboard, it doesn't matter what card numbers you have--they'll preboard you. IF you can't preboard, then you are boarded in groups of 30, starting with boarding cards numbered 1-30, then 31-60, etc. There is a line formed with passengers with 1-30 and another with 31-60, etc. NOW, they don't board with cards 1,2,3,4,5.....Anyone with a card between 1-30 can be first in line (example, the 1st person in line may have card #15, the 2nd in line may have #27, the 3rd person may have #6, etc. ). If you have a card between 1-30, all it means is that you'll be one of the 1st 30 people onboard. If your card number is between 31-60, you will be one of the 1st 60 passengers boarded, etc.
I haven't had any trouble getting seats for my family (4 altogether) when we've held boarding cards less than 60 (I've never a higher number boarding card when travelling with the family).
Also, SW is pretty good about moving people to accomodate families--especially coming out of Orlando. During busy seasons they fly hundreds of kids a day.
As I said earlier, you probably won't have any trouble preboarding on your flight TO Orlando, but you might want to check at the gate when you get there so you know what to expect when you're leaving.
Sorry to be so long.....