Difficult to comment with authority, as I was not there, but the description of the incident does not make the BA steward look particularly good at customer relations.
For example, the wife could have taken the middle seat. or the boy could have been moved.
Then again, there are two sides to a story.
Had this happened to me, I would have declined to move; if the plane misses it's slot, blame the airline for not having the gumption to implement it's own policy when allocating seats.
I had a 10 year old boy (UM) next to me on Air Malta last year, no problems. Air Malta placed him in 1D, where the crew could keep a close eye on him.
I was totally kn@ckered from playing board and card games by the time we arrived at Heathrow. A very charming and enthusiastic lad, whom it was a pleasure to sit next to.