First off, the flight will be full. MHT to BWI is full most of the time. Do check in 24 hours prior.....Do Not Wait. Family boarding will be before the B boarding group. There will be plenty of seats mid cabin back because there will be no "Through" passengers on board to BWI. All aircraft seat 6 across with the center aisle dividing the row except for the emergency exit row which will have 5. If the kids are under 15, this is not an issue because it is illegal for them to sit in the exit row. There is no stampede anymore. The Southwest website has a video on how the boarding process works now. It changed 2 years ago. Lastly, ask the pilots if the kids can come up for a picture. We love having kids come talk to us. Have a good trip and Thanks for flying Southwest!