It is fair to assume that any volume with space for a discussion of the “crisis” in mid-20th-century Soviet children’s theater is aimed at a specialist audience. That said, if the subject of “Children’s World” (Yale University Press, 736 pages, $45), Oxford professor Catriona Kelly’s immense, imaginative, and thoroughly researched new book — a history of child-rearing in Russia between the twilight of the tsars and the fall of Gorbachev — is somewhat academic, her prose style is not. She writes clearly, keeps her use of pedagogic jargon to a minimum, and even leaves room for occasional flashes of dry, [...]