Day at the Beach by Carol Emshwiller

Day at the Beach by Carol Emshwiller (F&SF, August 1959) begins with two (hairless) parents discussing, over their oatmeal, the dangers in commuting to the city to get food. Thereafter we get other hints that this is a post-holocaust or post-Collapse future when a discussion about a possible trip to the beach has mention of the boardwalk having been used for firewood and, when the couple’s three-year old comes in from outside, he is described as having down growing along his backbone (the woman wonders “if that was the way the three year olds had been before”). The child also bites a small chunk out of his mother’s shoulder when she chastises him for knocking over his oatmeal.
After this setup the couple decide—partly because they think it’s Saturday, partly because it’s a nice day—to go to the beach: they fill the car with only enough petrol to get there, and take a can’s worth for the return trip (which they plan to hide while they are on the beach). They also take weapons: a wrench for her, and a hammer for him.
On the drive there they see only a solitary cyclist and then, when they get to the beach, no-one at all. Later on, however, three men appear and threaten them, saying they want the couple’s gasoline. There is then an altercation during which the husband kills the leader with his hammer and the other two run off. Then the couple realise that the child has disappeared.
The remainder of the story sees the couple searching for the kid, and the husband eventually bringing him back. At this point the wife notes that they have time for one last swim (this with the attacker’s body still lying nearby). Then, on the way home:

He fell asleep in her lap on the way home, lying forward against her with his head at her neck the way she liked. The sunset was deep, with reds and purples.
She leaned against Ben. “The beach always makes you tired,” she said. “I remember that from before too. I’ll be able to sleep tonight.”
They drove silently along the wide empty parkway. The car had no lights, but that didn’t matter.
“We did have a good day after all,” she said. “I feel renewed.”
“Good,” he said.
[. . .]
“We had a good day,” she said again. “And Littleboy saw the sea.” She put her hand on the sleeping boy’s hair, gently so as not to disturb him and then she yawned. “I wonder if it really was Saturday.”  p. 174 (The Year’s Best SF #5, edited by Judith Merril, 1961)

This is an effectively dystopian piece, but its impact will probably be blunted for most readers by the many similar tales that have appeared since. I suspect, however, this story was notably grim for the time, and it foreshadows later new wave stories.
*** (Good). 4,100 words. Story link.