Complex sentence: The family first searched the area for Ben, assuming he had wandered off or their teenage son Stephen had taken him out on his moped but, when there was no trace found of the little boy, the police were notified.
Split into simpler sentences: The family first searched the area for Ben, assuming he had wandered off, or that their teenage son, Stephen, had taken him out on his moped. When there was no trace found of the little boy, the police were notified.