We have flown twice on SW, from CT to Orlando, with kids under 5 and have always been able to pre-board. 24 hours before our flights I did print our boarding passes, just in case, but it was not necessary. The CM's at our hotel also printed out our passes from the front desk without a problem, you just need to have your conf #. Have a safe trip!
With the money we save I am sure we will buy a dvd player and also bring our laptop (we always bring that anyway).
and it's been a real eye opener for us as well. The show makes it appear that they overbook A LOT. lol. We usually don't fly SW because we have found better fares on other airlines. I personally really dislike their non-assigned seating policy. Every time I walk by a SW terminal at the airport, it has just seemed so chaotic there to me. But I hear they are well-liked overall, so to each their own! I hope you have a great trip.