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Lucia, six (middle), pictured with her parents, was stabbed to death by her nanny along with her little brother Leo. Facebook

A horrified mother returned home to discover her two children murdered in the bathtub and the nanny stabbing herself in the neck, the Supreme Court in Manhattan heard.

Marina Krim told the murder trial of Yoselyn Ortega how she found her daughter Lucia, six, and son Leo, two, stabbed to death in the family's luxury apartment in New York on 25 October, 2012.

She had returned to the residence with her third child, then-three-year-old Nessie, after Ortega failed to turn up with her other two children at a designated meeting place.

Krim frantically searched every room, thinking, "its like a total horror movie," before finding the bathroom drenched in her children's blood.

She ran out of the apartment screaming, describing it as "a scream you can't even imagine is inside of you."

Heartbroken Mrs Krim was forced to describe the gruesome killings on the first day of the 55-year-old child carer's murder trial on Thursday.

Ortega has pleaded not guilty by reason of insanity. She faces two counts of first-degree murder and two counts of second-degree murder.

The baby-sitter is accused of plotting the murders and waited to be alone with the children with the apartment's kitchen knives, Assistant Manhattan District Attorney Courtney Groves said.

Krim told the court: "I just wanted to wake up from this nightmare that I knew wasn't a nightmare. It was real."

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A woman prays in front of the building where two children were stabbed to death allegedly by their nanny in a family's Upper West Side apartment on October 26, 2012 in New York City. Getty

Her eldest child Lucia, known as Lulu to the family, fought back before succumbing to her 20 to 30 stab wounds. Her little brother Leo had five wounds, Groves said.

The prosecutor said that Ortega waited until Mrs Krim returned home to stab herself so that she could see the distraught mother's reaction.

The court heard that the nanny had been hired two years before the killings by Mrs Krim and her husband, then-CNBC executive Kevin Krim.

Groves said that Ortega resented Krim and her family because she was the mother "she could never be."

Ortega was said to be overwhelmed with financial concerns after recently bringing her 17-year-old son Jesus from the Dominican Republic and enrolling him in a private school so that he didn't have to repeat the 11th grade.

She was "enraged" when Krim offered to find her more work, Groves added.

It emerged in the trial that the two women had clashed in the past, as Krim testified that they fell out over an offer by a friend of the family to hire Ortega as a household helper. She said that she confronted the babysitter on the matter, though few details were enclosed.

Krim and her husband have gone on to have two sons, Felix, now four, and Linus, two.