We've had B several times and still were able to sit together. You will probably need to go further back in the aircraft which is really no big deal. SW seating is three and three across, so you may not get all three across, but, rather two and two across, or two and two behind. I wouldn't attempt to go back and change it because if anything you would be changed to a C not an A. Once the A's are gone they're gone. How old are your kids anyway? If you have any under 4 you should be preboarded anyway so the boarding pass becomes a moot point. Once in awhile if the flight is booked solid, or even overbooked they'll suspend preboarding, especially if it appears the preboard group is the majority of flight, but, I've only seen that happen once. Don't worry about it......SW flight attendants are very good at making sure you'll be able to sit with your kids. Enjoy your flight!! I just saw your daughter is only 3......you should be preboarded no problem!!