While every person is a child of Heavenly Father, we can also become children of Christ through faith and obedience (see Mosiah 5:7). By virtue of his faith in Jesus Christ and his willingness to obey, Joseph Smith was also a son of Jesus Christ.
"Follow the example of Joseph Smith and the pattern of the Restoration. Turn to the scriptures. Kneel in prayer. Ask in faith. Listen to the Holy Ghost. Learn that your name and needs are known by our Heavenly Father, just as Joseph's were" (Robert D. Hales, "Receiving a Testimony of the Restored Gospel of Jesus Christ," Ensign, Nov. 2003, 31).