"Joseph Smith, the Prophet and Seer of the Lord, has done more, save Jesus only, for the salvation of men in this world, than any other man that ever lived in it." The reason we teach the the martyrdom of Joseph and Hyrum Smith can help the children feel gratitude for the testimonies and sacrifices of these great men.
Hearing about their sacrifices can strengthen the children’s faith and their desire to be true to their testimonies.