Thank you HKGer, for your response. Since no one had given an opinion, I called in any case, and found out that "M" fares on domestic are allowed to select preferred seats. Now since one of the travelers is 8, they can't sit in the exit row, but they have a window and an aisle, since all seats together (other than the exit rows) were taken. Hopefully the flyer in the middle will be happy enough to take the aisle (or the window) unless he really really wants the middle seat, which is ok with both the 8 yr old and her mommy.