Superman discovers a wife and child he knew nothing about in the Dorfman, Swan and Abel story in issue 218 (July 1969).
Superman and Lois Lane spot an infant in a Superman costume, playing with diamonds. They find the mother, who insists that she is Superman’s wife, and the child their son, who created the diamonds himself, out of coal. Superman is certain that she is wrong, but she shows him a wedding certificate and photo album, even an engraved ring. The boy has all of Superman’s powers, and when Superman exposes the child to kryptonite, he begins to turn green.
From the information the woman, Larissa Lenox, gives them, Lois Lane theorizes that Mr. Mxyzptlk used his magic to remove Superman’s memory of his marriage and child, and then just sort of hovers around the fringes of the story, stressing over Superman having a wife.
Superman spends a while racing after his son, whose desire to play with full sized versions of children’s toys causes some chaos.
But Superman is really just playing along to spring his trap. The child is really Mr. Mxyzptlk, and Larissa nothing more than a magical construct. Superman suspected something was up from the start, as the diamonds the child had supposedly created were polished and faceted. When Superman crushes coal into diamonds they come out as rough hunks of diamond. The kryptonite he exposed the child to was fake, there was no way it should have caused a reaction.
It’s kind of unfortunate that the story cues us in to Mr. Mxyzptlk so early, as I doubt I would have figured it out without that.