If you check in online you can get an earlier boarding group and not have to get to the airport too early. If you're checking luggage I'd still suggest at least 90 minutes before your flight time. If you don't check in online, you might want to plan to be there the full two hours early to give yourself a better chance of being in the first group to ensure that you get seats together. With just two of you, your chances of not getting seats together is pretty slim unless you're really late (or everyone else is really early), so you shouldn't have too much to worry about in any event.
To be clear for other DISers, I'm pretty sure that the SWA cut off age for pre-boarding is children under 5, not children under 12. Many other airlines don't even offer pre-boarding at all anymore.