I assume you're asking about Resort Airline Check-in.
Sorry, Southwest does not participate in Resort Airline Check-in.
If you're flying on Southwest (or any other airline that is not part of RAC), the Disney's Magical Express driver will load your bags into the luggage hold of the motorcoach and return them to you when you arrive at the airport. You'll then check in and check your bags at your airline's counter (or with a skycap), just as if you had arrived by taxi, rental car, livery service, or private car.
It's customary to tip most people who handle your bags (such as the Magical Express driver, airport porter, or a skycap). One does not tip the airline employees at the airline counter or the TSA employees.