I should have said that this actually wasn't a problem anyway as these were not lap children. But, obviously, originally the agent incorrectly assumed that they were.

It IS a rule that lap children can't be across the aisle from each other it all seats in that row are occupied and also that there can't be a lap child in the last row of an Alaska 737-800 if all seats are in that row are occupied. Flight attendants are to watch for these two instances and move people before takeoff if either does happen.