I don't know why they split up our particular party, either. I can only assume it was to keep other parties together. So, we are a party of three. The three other parties who share our rows were kept together. That gives the airline a 75% rating on keeping parties together. (I don't know this for a fact, I am just supposing).
It worries me, too, about having to sit away from my son. He is afraid that somebody might try to kidnap him. I just explained to him to stay in his seat until mom or dad can get to him. Another time when we were split 2 on one side, one on the other, we just happened to meet our rowmates in the waiting area before the flight. This was a mom and a little girl my son's age. They played together before the flight, and were friends by the time the plane took off. The mom wisely decided that it would be better to let my 6yo son stay in the middle seat of the row instead of my 6'2", 230lb husband.
So, it turned out OK, but it was a happy coincidence, and I have dealt with many people who are no where near so agreeable.