PARK CITY, Utah — Robert F. Kennedy feared the Mafia would try to blind his young children in an acid attack to deter his investigation into labor racketeering, his widow, Ethel, reveals for the first time.

Speaking out in a documentary by her daughter Rory premiering today at the Sundance Film Festival, feisty matriarch Ethel Kennedy recalls a scary threat when her husband was counsel to a Senate subcommittee probing organized crime’s control of labor unions.

“There was a journalist for the New York Post they had thrown acid in the face of,’’ Mrs. Kennedy, 83, says, referring to crusading columnist Victor Riesel, blinded by a hood linked to the Mafia in a shocking 1956 attack. “We were told they were going to do the same with our children.’’

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Eldest daughter Kathleen, one of several siblings also interviewed in the film, recalls, “We couldn’t leave [school] with the other kids at the end of the day. We had to wait in the principal’s office to be picked up.’’

The documentary “Ethel,’’ which will play on HBO later this year, offers an extraordinary look into the private lives of a celebrated family that was at the center of some of the most famous events, triumphant and tragic, of the 20th century.

Asked about her husband’s 1968 assassination, Ethel says to her filmmaker daughter: “When we lost Daddy . . .” then stops, pain written on her face.

The family credits devout Roman Catholic faith with getting them through almost unendurable losses. Following the assassinations of her brother-in-law, President John F. Kennedy, in 1963, and her husband, Ethel later lost two of her 11 children — one from a drug overdose and the other in a skiing accident.

“I wake up every morning and I think of Daddy [Robert] up there with Jack and [their older brother] Joe and my parents,’’ Ethel tells her daughter Rory, the youngest of her and Bobby Kennedy’s 11 children. A noted documentary filmmaker, Rory was born six months after her father was fatally shot after winning the California Democratic presidential primary.

“When the rest of the world was grieving,’’ her mother told the children their father was in a wonderful place, says Kathleen Kennedy. “Her faith is so strong — that’s caused her to get through everything [including] losing [sons] Michael and David.’’

As Ethel puts it: “Nobody gets a free ride. “You have your wits about you and dig in because it might not last.’’

When JFK appointed his brother as attorney general, his outspoken sister-in-law quickly emerged as one of the more colorful members of the extended Kennedy clan.

At one point, she was charged with horse theft — then a hanging offense in Virginia, where the family lived on a farm — after she rescued a neighbor’s maltreated horses.

Ethel was acquitted, but JFK asked her to tone down her famous parties — after press reports of a soiree where “all the members of his cabinet were thrown in the pool,’’ Ethel’s son Joseph Kennedy recalls.

After JFK was assassinated, Ethel says, “It was like Daddy lost both arms. It was just six months of blackness.’’

The documentary includes extensive home-movie footage of the family that’s never been shown publicly — including a striking image of a stricken Bobby Kennedy sitting in quiet contemplation on the side of a road.

According to Ethel, it was very difficult for her husband to seek office for the first time, successfully capturing a US Senate seat in New York in 1964.

“Whereas Jack was a born orator, nothing came naturally to Daddy, he had to struggle for everything,’’ she says.

Rory Kennedy says HBO, where she’s made films about AIDS and human rights issues for more than a decade, had long urged her to do a film about her family, but she resisted.

“It’s not in my comfort zone, and I assumed my mother wouldn’t want to do it,’’ Rory tells The Post. “But she sat down with me for five days and answered every question in the book.’’

One of her favorite stories is that when Robert was attorney general, Ethel would take the older kids to watch sharpshooters in the basement of the FBI building (the bureau fell under Robert Kennedy’s jurisdiction).

Kathleen says in the documentary, “One day she noticed a suggestion box. She took out her signature red pen, wrote, ‘Get a new director’ and put it in the box.’’

Rory Kennedy — who will be joined by her mother and about 25 other family members for the premiere in Park City, Utah — adds that longtime FBI director J. Edgar Hoover, no fan of his nominal boss Robert Kennedy, quickly discovered what happened.

“By the time [Mom] got to my father’s office with all the kids, Daddy had already gotten the note from an irate Hoover,’’ she says.

lou.lumenick@nypost.com