Boarding cards are assigned a letter of A, B, or C. They are distributed on a first come first serve basis. There are approximately 40-45 of each letter depending on the configuration of your exact plane. A are distributed first, and so on. A will board first, B will board second, and C will be the final group to board. You should have no problem trying to find 2 seats together. Have one parent sit with each child. Many times on MCO Flights they do not pre board families because the plane is literally the Disney Family Express. They will only preboard one parent and an infant if the child is in a car seat and has a seat purchased for the infant. Get to the airport with plenty of time to check luggage and clear security and you will be fine. SWA only flys planes that have a 3 by 3 configuration. So your best option is to fly 1 parent and 1 child configuration. As long as you receive A or B boarding cards you will have no problems finding this configuration. Please don't believe the horror stories people recite about getting to the airport 4 hours early. It really isn't necessary. Avoid flights at peak times for business travel (Early Monday morning and evening flights on Thursday and Friday nights) and you will be fine. SWA is very kid friendly and has a terrific ontime record.
I highly recommend SWA especially if any other option requires a layover or changing planes. The last thing you want is to entertain a child during a layover.