There's no way to predict what boarding number you'll get if you check in an hour later. Here's the breakdown:
A1-A15: These are reserved for Business Select passengers who pay the highest fares. I fly Southwest a lot and have rarely seen all 15 spots taken
A16-A60
B1-B60
C1-whatever the last number is...
I think there are 137 total seats, though. Remember if you get a B pass that the entire A group will be in front of you, plus families with children under the age of 4 - they are allowed to board between As and Bs if they don't already have A boarding passes. If you don't mind splitting up with 1 child with you and the other child with your husband (or have the teens sit together if they are okay with that), you shouldn't have any trouble as long as you get a pass better than about B30-B40. If you want 1 row of 3 seats and another on the aisle across from it, you'll need to be in the A group or very early in the Bs.
For $10 per person each way, you can purchase Early Bird Check-In and you don't have to worry about the 24-hour thing. You will automatically be assigned boarding numbers at 36 hours prior to your flight. This will put you behind any Business Select passengers but ahead of everyone who checks in at 24 hours. And you don't have to do anything special other than fork over $10 per person.

I use EBCI every time now and I wouldn't have it any other way - no fooling around making sure I'm online at whatever time is needed.
And to answer your last question, for your return flight, you figure the 24-hour window based on local time, which is Orlando time in this case.