A Manhattan mom out running errands returned to her luxury Upper West Side apartment last night and walked into a nightmare — her two young children had been stabbed to death by their nanny, who then slit her own throat in a suicide attempt, police sources said.

The mother, Marina Krim, arrived at her West 75th Street home at 5:34 p.m. after nanny Yoselyn Ortega, 50, failed to meet her for swimming lessons with the kids she was watching — Leo, 2, and Lucia, 6.

The building’s doorman told Krim the nanny hadn’t left, and she went upstairs to find the reason for the hold-up. She found Ortega slashing her own neck, with both her wrists already slit and bleeding.

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Krim rushed to the bathroom of the second-floor apartment and found Leo and Lucia in the tub, lying in a pool of blood with stab wounds all over their bodies.

“Something happened to my kids!” screamed the sobbing mom, who wrapped the nanny’s neck with a towel.

The doorman, Glen Loody, heard her and called 911. “She was crying. She was screaming,” he said.

The kids appeared to be breathing as they were rushed to St. Luke’s Hospital but were pronounced dead on arrival.

Krim and her surviving middle child, Nessie, 3, — who did not see the carnage — were also taken to St. Luke’s, where Krim had to be sedated.

Ortega, who was is in police custody, was taken to New York-Cornell Hospital, unresponsive, in critical but stable condition. Police believe she may have taken pills, a law-enforcement source said.

She has no criminal record and had not yet been charged with a crime late last night, sources said.

The children’s paternal grandmother, Karen Krim, said Ortega came into the family’s life about a year ago.

Up until then, Marina Krim was a stay-at-home mom and cared for the kids exclusively. The Krims decided to hire a nanny when Leo was born.

Marina and Kevin Krim had been extra careful in hiring Ortega — and even spent nine days with her family in the Dominican Republic beforehand.

“Kevin told me that she was a nice girl,” the grandmother told The Post, sobbing. “How could she do something like that?

“The children were angels.”

Ortega’s niece, Katherine Garcia, 28, said her aunt had seemed a little off and was “acting kind of nervous lately.”

“This is just shocking,” she said. “She loved those kids. I don’t know what would make her do this.”

The children’s father, CNBC executive Kevin Krim, was returning from a business trip to San Francisco and was notified once he arrived in the city last night, a source said.

Police took him to St. Luke’s.

Marina Krim diligently posted thousands of pictures chronicling her beloved children’s lives in an online journal — from apple-picking and family vacations to strolls down Broadway.

In her latest entry, posted just three hours before finding Lucia and Leo dead, the loving mom wrote about her son learning to put words together.

“Leo speaks in the most adorable way possible,” Marina Krim wrote, describing her mood at the time as “amused.”

Neighbors described the Krims as a happy, young family.

“It’s horrifying,” said one resident, who met the nanny for the first time Wednesday.

“She was very quiet — she just stared ahead.”

Another neighbor said, “I just don’t understand. It’s so senseless.

“A nanny would be the last person I’d think would ever do something like that. I’m in shock. Total disbelief.”

Originally from the West Coast, the Krims moved to the building, called the La Rochelle, a couple of years ago.

Just last month, all three kids were at their aunt’s wedding.

Leo was a ring bearer and his sisters were flower girls.

The family pet was a retired racing greyhound named Babar.

This morning, the CEO of Comcast, which controls 51 percent of CNBC parent company NBCUniversal, expressed his sympathy for Krims during the cable company’s earnings call.

“The father of the children was part of our CNBC family,” CEO Brian Roberts said as he wrapped up the call, according to The Hollywood Reporter.

“So, I just wanted to say on behalf of all of us at CNBC and Comcast and NBCUniversal and many others around the nation how touched and sad we are by this unspeakable act. And we will do anything we can to support the family in [this] awful time. Thank you, all, and we’ll go back to our jobs, but have them in our thoughts and prayers.”

Additional reporting by Linda Massarella, Daniel Prendergast, Kirstan Conley, Rebecca Harshbarger

and Antonio Antenucci