The Question and Answer section for The Monkey’s Paw is a great resource to ask questions, find answers, and discuss the novel.
The knocking at the door, presumably from their zombie son, is a detail in the conclusion.
Chess involves strategy, surprise, and risk. This both represents life and foreshadows the risks the White famly will take over the monkey's paw.
"He took something out of his pocket and proffered it. Mrs. White drew back with a grimace, but her son, taking it, examined it curiously." What is the MOST PROBABLE explanation for Mrs. White's initial reaction to the monkey's paw?
The paw was ugly and mummified.... not pleasant to look at.