One very good reason for offloading would be: If the child/father did this before departure - how would they make out for landing?
Some years ago (but I remember it vividly) and it was around 1997, I had such a small boy sit next to me in Biz on Swiss LHR~GVA. I was window, the boy in the middle and the mother on the aisle.
Departure was fine.
Flight was fine.
Belt up for landing? Full-on tantrum.
I had to help soothe the boy and physically restrain him. He eventually quietened down but it was very unpleasant. I recall thinking how patient I was as I HATE the sound of screaming children and it is one of the main reasons I chose never to have children myself. I did help because I wanted the plane to land.
Alaska did the right thing and I am VERY amused to read that the mother decided to let dad look after the boy. Did she know this kind of thing was going to happen? Does he sit in his booster seat and wear his seat belt in the car?