I can only assume that the Dad was not on the flight and the Mom was flying solo with the kids? Assuming that, I side with the mom - that's a lot to handle - BUT, in looking at the picture, the popcorn is pretty easily accessible. He stated she was 22 weeks pregnant (whose fault is that? Just sayin') but to me that works against his argument - she's in her 2nd trimester and really not "that" pregnant yet. She could easily get the few kernels that were there along the aisle, and yes, it wasn't that big of a mess either. If Dad is on the flight, maybe more parenting and less documenting would be in order.
OTOH, if the popcorn is between the seats especially near the window seat, forget it. The airlines put the seats too close together for anyone to reach down there while there are still passengers on board and everyone is supposed to be seated with seatbelts on. That's on the airline - provide more space between seats or use some of that added revenue to hire a cleaning crew between flights.