It was done to allow kids and parents to sit together. I did give up an isle seat once to allow a mother and her young daughter to sit together on a flight which was full since no one else would. I wished at the time they had pre-boarded.
To us it doesnt matter. As seasoned travelers, we will just make sure to get the first group of A boarding passes and board first.
And they will now have to do the same if they want to fly sitting next to Junior. ^