"When Mickey (who looks to be about three years old) enters the Night Kitchen, he loses his pajamas and spends much of the story fully naked.
Critics of the book object to Mickey's nudity (which explicitly depicts his penis and testicles ), with some librarians drawing little pants on Mickey with a marker, or diapers with correction fluid. Some also take a Freudian interpretation of events, with the nudity, free-flowing milky fluids, and giant (allegedly phallic) milk bottle. Sendak himself claims not to have been trying to be controversial; his decision to derobe Mickey was to avoid the "mess" that falling into the batter would make of Mickey's clothes. (Knowing Sendak, I am dubious.)
As a result, the book proved controversial in the United States on release and has continued to be so. The book has been ranked 25th place on the "100 Most Frequently Challenged Books of 1990-2000" compiled by the American Libary Association."
Note: I could not find these particular images anywhere on the Internet and had to scan my own copy of the book. It's a surrealistically wonderful book. Read it.