mylilnikita,
They do serve snacks on SWA--sometimes it is a very small bag of peanuts or a very small bag of mixed pretzels, nuts, etc. With a 5 year old--I would definately pack a more sustantial snack. (I'm thinking from ORF, your flight is just under 2 hours? )
I don't know if ORF is anything like BWI for the boarding process, but the folks at BWI are quite familiar with the boarding process and the flights are generally sold out whenever we fly, and our DS is 14, so no pre-boarding for us, either. We generally get there 2 hours early to get a boarding pass with a good boarding group (the link Gail gave you will provide more details). We have always been able to get our seats together (that is in 12 flights in the past 3 years).
I have even seen an occasion where a Dad and a young son have arrived after the boarding process is almost comlete (almost all the seats are full) and the SWA flight attendant sucessfully asked for volunteers to change seats so the two of them could sit together. But, I think the best thing to do is to arrive early and not take a chance.
BTW, I really get a big kick out of SWA. The flight attendants I have seen are probably the nicest I've seen on any airlines. Sometimes, they sing the safety message. Other times they tell jokes. ..."if you are traveling with small children---I'm sorry..." Sometimes, they do demonstrations. One time, we were flying to Buffalo and a couple of flight attendants unrolled toilet paper down the entire length of the aisle. They were demostrating one of the features of their new B737-700 model. The flight attendant flushed the toilet in the back of the aircraft and the toilet paper spanning the length of the aisle instantly disappeared.... The flight attendant in the front of the aircraft said, "I'm not going to tell you how we discovered this, but I just wanted to caution you that you don't want to be sitting on toilet when you flush it!" (

) Sometimes, the flight attendants play it straight, but one consistant thing is they have always been extremely nice.
Enjoy your flight.
-DC
