Radnitz died Sunday at his Malibu home from complications of a stroke he had years ago, said his wife, Pearl.
He went on to produce nearly a dozen feature films, often mining children's literature to make such movies as "Misty" (1961), based on the Marguerite Henry classic "Misty of Chincoteague," and "Island of the Blue Dolphins" (1964), which shared its name with the Newbery Award-winning book by Scott O'Dell.
"Island" was "the very model of what children's pictures ought to be but seldom are," Time magazine said in its 1964 review of the film. "Radnitz & Co. have provided sentiment without sentimentality and a moral without a lecture."
The filmmaker had a simple explanation for his approach to making movies aimed at children: "If you don't talk down to them, you'd be surprised how high they can reach," Radnitz told The Times in 1991.
His movies often featured a young person who must overcome adversity and were filmed inexpensively on location, using non-actors in secondary roles and indigenous music, Radnitz later said.
Nominated for an Academy Award for best picture, "Sounder" was his most acclaimed film. Based on William Armstrong's best-selling book about a black sharecropping family in the Depression-era South, it starred Cicely Tyson and Paul Winfield.
"Sounder" was "his ego trip," Radnitz recalled in a 1973 interview, "because everybody told me not to make it," and nobody thought it would find an audience.
"It was the first film that broke the mold of blaxploitation," he told The Times in 1996.
When Times critic Charles Champlin named "Sounder" one of the top films of 1972, he called it "beautifully acted, honest, angering and inspiring."
By 1973, The Times had proclaimed Radnitz "the only successful American maker of children's films outside the gates of Walt Disney films."
Robert Bonoff Radnitz was born Aug. 9, 1924, in Great Neck, N.Y., the only child of Fred and Lilyn Radnitz. His mother's family, the Bonoffs, started a laundry business that his father ended up running.
Raised on Long Island, Radnitz was an asthmatic child who regularly indulged in Saturday movie binges in New York City. Filmmaker Stanley Kubrick said Radnitz was "the only person who has seen more old movies than I have," People magazine reported in 1976.
At the University of Virginia, Radnitz studied English and drama. After earning his degree, he stayed to teach English literature.
Eventually, Radnitz apprenticed with influential theatrical director Harold Clurman, according to his family, then produced two Broadway plays, "The Frogs of Spring" and "The Young and the Beautiful," between 1953 and 1955.
After he came to Hollywood, he became a script consultant at 20th Century Fox, which financed his first feature.
By 1970, he had entered into a partnership with toy maker Mattel to produce family films, releasing "Sounder" and the Appalachian drama "Where the Lilies Bloom" (1974), among other movies.
His movies were "marked by a happy lack of condescension," Time magazine pointed out in 1969 while writing about "My Side of the Mountain," his film about a Canadian boy who runs away to nature.
The producer's final feature film was "Cross Creek," a 1983 drama that unfolds on a Florida bayou. It was nominated for four Academy Awards.
A devoted tennis player, Radnitz almost always wore his tennis whites, even to business appointments.
Soon after moving here, he bought his beachfront home in Malibu. Every morning at 6 a.m., Radnitz swam naked in the ocean "rain or shine," he told The Times in 1973.
The swims continued until he had a stroke in 1996, which also ended his career.
Until then, he had been "a robust, hale and hearty type of guy," said his friend, Keith Robinson, "always in charge and a great storyteller."
When asked why the license plate on his Mercedes-Benz convertible said "CALM," Radnitz would invariably reply, "It is a state I hope one day to achieve."
Pearl, his wife of 23 years, is his only survivor.