NEW YORK (CBSNewYork) – A father was driven to tears Tuesday in federal court as he faced the nanny accused of killing his two young children inside their Upper West Side apartment more than five years ago.

Before the murder trial began, Kevin Krim and his wife, Marina, talked on Facebook about having to relive the horror of losing 6-year-old Lulu and 2-year-old Leo.

“This trial will be very hard for us and for a lot of you,” he said.

On Tuesday, Krim took the stand in the murder trial of his former nanny Yoselyn Ortega. It was his first time seeing her since before she allegedly killed the kids inside the family’s apartment on October 25, 2012.

More: ‘You’re Evil And You Like This’: Mother Rages In Court During Trial Of UWS Nanny Accused Of Killing 2 Children

On that day, he was returning from a business trip. Upon landing at John F. Kennedy International Airport, his father-in-law called him with the terrible news, saying, “two of your kids are dead.”

“It’s the worst thing imaginable,” Krim said with a shaky voice. “I was hoping it was a nightmare, but it wasn’t.”

He said police escorted him to the hospital, where he learned of the disturbing way Ortega had confessed to killing Lulu and Leo.

Krim shook his head in disgust at Ortega in the courtroom.

He said she never showed signs of mental illness while working for him, saying “she looked totally normal.”

Krim said he insisted on seeing the children at the hospital so he and his wife could say goodbye.

“I got down on my knees and I said, ‘I’m sorry.’ I said I loved them, kissed them and said goodbye,” he said, testifying through tears.

Krim left jurors with his beautiful memories of his children, describing his son Leo as awesome with big acorn eyes that saw everything. He called Lulu his little tomato, an old soul and the love of his life.

The prosecution rested its case Tuesday. Ortega’s defense begins Thursday.

If convicted, she faces life in prison. If found not guilty by reason of insanity, she could spend the rest of her life in a mental institution.