Lessing's story "Through the Tunnel" was published in 1955, one year before she was declared a prohibited alien in her former home of Southern Rhodesia (now called Zimbabwe). While the allusions to apartheid—an often violent policy of segregation and political and economic discrimination--are fairly subtle, the mention of the dark boys who are far from the tourist beach hints at this segregation. This racial separation is probably the cause of the dark boys' surprise that Jerry wants to interact with them, as well as their quick departure.
That they are physically superior to Jerry may be Lessing's way of suggesting the inequity of apartheid, and that Jerry finally swims through the tunnel wearing goggles also suggests that he needs more protection than do the other boys because he is softer, being raised by a mother who dwells on a higher level of society. This contrast between the lives of the native boys and Jerry is evinced in this passage about the other beach away from that of the tourists:
It was a wild-looking place, and there was no one there; but she said, “Of course, Jerry. When you’ve had enough, come to the big beach. Or just go straight back to the villa, if you like.”
Another aspect of the story that may allude to Lessing's own experiences is the placement of a widowed mother for Jerry. For, in the early fifties, many women had to raise their children by themselves since their husbands had been killed in World War II.