THE KING OF CHILDREN <i> by Betty Jean Lifton (Farrar, Straus & Giroux: $24.95; 353 pp.) </i>
Despite the story’s melancholy ending (Janusz Korczak and the Jewish children in his orphanage died in the Treblinka concentration camp), Lifton does a fine job of evoking Korczak’s lively mind, his generous heart.