NEW YORK — Maybe Sid Fleischman should take more showers. "There's a call from Chicago," his wife, Betty, told him while he was sudsing away the week before last. "I don't know anybody in Chicago," he called back. After he had dried off, the Santa Monica writer called back and learned the telephonic missive had not been a plea to speak at some conference or convention. ("Children's writers are called on a great deal for speeches," Fleischman explained.) Instead, it was the American Library Assn., wanting to inform the 66-year-old scribe that he had just won the John Newbery Gold Medal for children's literature. "If there was pizzazz to it, it would be the Academy Award of children's literature," Fleischman said, overjoyed that after "I think it's 34" books for young people, he had finally earned top honors. "You can imagine, it had to go through brain cells when they told me I'd won," Fleischman said. "You don't expect something like this to come. Then I realized, my God, it's happening to me." Illustrated by Peter Sis and published by Greenwillow Books here, "The Whipping Boy," Fleischman's winning tale, takes its inspiration from the royal houses of Europe, "where," said the author, "the prince, the heir to throne, couldn't be punished. So if they had a rotten prince, they installed a commoner off the streets, and he took the punishment for the prince." As the onetime vaudeville magician and former writer of adult nonfiction recalled, "The injustice of it enraged me. The lunacy of it! So I couldn't let go of this idea, even though I couldn't think of a way to tell the story." Fleischman struggled with the story for 10 years, until finally "I realized the story wanted to write itself a little longer, so I just let it go. Once I took off the straitjacket I had put on the story, it just exploded, and I had great fun writing it." Credibility is one of the fringe benefits of a prize like the Newbery Award, Fleischman said, for winning it focuses more attention on the entire genre of children's literature. "I am absolutely persuaded that the best books being published today are in the children's field," he contends. Also, Fleischman's victory is a particularly happy occasion for his family, since several years ago his son Paul picked up the silver Newbery for his book "Graven Images." Said Paul's father, noting that, in fact, he wrote his first children's book as a vehicle to amuse the three Fleischman offspring: "Our styles are completely different. His work is very Hawthornean. My stuff is rather broad and comic. But I deal with serious things, as I suppose all comic writers do."