In the part of the movie where the fasten seat belt sign comes on and dash and violet put them on. then go to c*ck pit and the mom does a barrel roll. and the kids and the back are flying around.
I didn't pay that much attention but I am pretty sure the plane banks before they are actually buckled. If you watch Violet closely, she barely even gets the seat belt around her before the plane drops, let alone enough time to have it buckled. Who know's? Maybe they have some of their dad's strength and accidently broke the seatbelts...haha.
Oh, come on. Watch the scene a little closer. It's supposed to be funny. The missiles are coming. Mrs. Incredible presses the SeatBelt button, and the kids try to put them on, but are yanked out of their seats before they can by the barrel roll.