Something similar happened when my wife and I flew to Rarotonga on Air New Zealand last year. The seats we were allocated on check-in were 8 rows apart. We managed to get them changed so that we were siting directly across the aisle from each other. Once we borded we discovered a mother travelling with 3 children were sepearted all over the plane. My wife swapped with one of the children, and another passenger swapped with another one. Eventually we managed to get the family together.
The cabin crew were marvelous in helping out, but this situation should never have happened in the first place.