In "My Parents," Spender explores the theme of the isolation and alienation he felt from the other boys his age in his neighborhood. They were poor, rough boys dressed in torn, ragged clothing who played in the streets. His parents, he says in the opening line, "kept him" from such children—except that they didn't, not really.
After all, these strong, tough boys bullied Spender physically and frightened him. They also mocked him, imitating his lisp and pointing at him. The poem shows Spender's perception that is was "them" versus "him."
In the last two lines of the poem, Spender says that when the boys threw mud, he looked the other way, "pretending to smile." With the smile, he was trying to get along with them and make a connection that would allow him to feel less isolated and alienated from these lads his own age. However, even though he wanted to "forgive" them, they never smiled back, so there could be no connection. These last two lines therefore reinforce the poem's theme of isolation and alienation.