Carl Kent is the son of Superman and Larissa Lenox in Superman #218 (July 1969).
A woman shows up claiming to be Superman's wife, and that her child is his son. She shows him a wedding certificate, photo album and ring, so Lois theorizes that Mr. Mxyzptlk magically removed Superman's memories of his family. It turns out that Carl is Mxy in disguise, and Larissa is only a construct. Superman tricks him into reciting his name backwards.