A two-year-old boy walks through the park with his mother, telling every stranger the same happy news: his father is coming home from the war today. He believes it completely, because a letter promised it. The simple, bouncy rhymes and his cheerful chatter make him sound like any excited child.
But the poem turns on the mother. People look into her green eyes and see "the pain and regret of the lies." The poem doesn't say outright what has happened, but the final lines make it clear enough: the father will not be coming home, and the boy will keep waiting for something that "would never come." The word "lies" suggests she has been unable to tell him the truth, perhaps to protect him.
The mood is heartbreaking because of the gap between what the boy knows and what the adults understand. His hope is sweet and sad at once, and the strangers, like the reader, can only watch.