Why does book starts with parents dicussion?

Roald Dahl begins the first chapter by talking about parents. Most parents will brag about their children no matter what they do, even if the child himself is not necessarily worthy of praise. However, there are also parents who do the opposite—they show little interest in their children. This is the case with Mr. and Mrs. Wormwood, who look upon their daughter Matilda as "nothing more than a scab," even though they dote on their elder son, Michael.

The way the Wormwoods ignore and loathe Matilda is jarring for readers, because the common perception is that all parents love and care for their children. Dahl even acknowledges how unsettling this is in the first chapter, beginning the book with an account of the way most parents should treat their children. This, juxtaposed with Matilda's parents' poor treatment of her, helps to create the feeling among readers that something here is not right.