sbm, in the situation that you describe it sounds like the if the pair travelling together is say a mother and her minor child (you mentioned one of them was a minor, right), it sounds perfectly reasonable that CO should rearrange people if necessary to ensure that they sit together. Obviously shouldn't have moved them to an exit row, but it's understandable that exceptions are made for folks in that situation.
sorry but I disagree with You. CO can ask someone who alreay chose a seat if theyd move, but CO shouldnt simply go and move anyone who chose a seat, maybe I was traveling with someone and for whatever reason needed to be next to them and we are on different PNRs.