I would pack 2 large suitcases and a big carry on, but include a large empty duffel bag that you can use in case you come back with more stuff (which you will, or at least we do, stuffed animals take a lot of space LOL!). You get one checked bag for free per person with southwest, so you don't want to risk having to pay for extra luggage on the way back. I am a fan of packing a decent size carry on with at least one change of clothes for everyone, and anything you think you may need in the first two days, just in case your luggage gets lost.