An out-of-control driver killed two little kids in front of their horrified moms in a Brooklyn crosswalk Monday — with one tot dragged about 100 feet down the block while still in his stroller, authorities said.

The stricken mother of the ­1-year-old boy in the stroller knelt beside her son’s body on Ninth Street near Fifth Avenue in Park Slope and frantically tried to revive him, a witness said.

The mom was identified by law-enforcement sources as Lauren Lew, 33, of Park Slope.

The little boy, Joshua, was the older of her two sons, said Lew’s grandmother.

“He’s a darling little boy,’’ the grandma said by phone, her voice trembling. “I still can’t believe it.’’

The other child killed in the 12:40 p.m. horror was a 4-year-old girl accompanied by her pregnant mother, Ruthie Blumenstein, 34, of Park Slope, officials said.

She “was bleeding from her head. They had to cut her coat off,’’ bystander Leah Finnegan said of Blumenstein.

Blumenstein’s daughter “was flat on her back, and when the ambulance came, they were giving her CPR,’’ a witness, June Clark-Smith added.

“I ran to the [mom], telling her, ‘Please, Miss, stay down.’ . . . She tried getting up on her knees and fell back down.’’

Blumenstein was in critical but stable condition in the intensive-care unit at NewYork-Presbyterian Brooklyn Methodist Hospital. Lew was hospitalized in stable condition, officials said.

Dorothy Bruns, 44, of Staten Island was driving her white Volvo sedan on Ninth Street crossing Fifth Avenue when she slammed into the mothers and children, police said. She then veered into a parked vehicle, causing a chain reaction that damaged at least three more cars, law-enforcement sources said.

A baby’s shoe was among the debris let scattered in the street — and Joshua’s stroller was crushed under the Volvo’s back wheels.

Bruns told police she had a medical condition, and she was taken to NYU Langone Hospital-Brooklyn, sources said.

A super at Bruns’ apartment building said, “I hope she didn’t have another seizure.”

“She was telling me that two months ago she had one when she was driving over the Verrazano Bridge and she stopped the car,” the super said, adding that Bruns has a teenage son.

“She was hooked up to machines for a while, and she was on medication,” the man said. “This is really tragic. I know she is hurting because she has a child herself.”

Law-enforcement sources said there is video surveillance from the scene, and it was unclear whether the driver had the light.

Bruns told cops she was creeping up a bit at the intersection in anticipation of the red light changing and then accidentally hit the accelerator while the moms and kids were in the crosswalk, sources said.

Police sources said Bruns would not be held and was free to go, at least for now.

Additional reporting by Nick Fugallo and Shari Logan