The woman entered the building, La Rochelle, on Manhattan's Upper West Side, with her 3-year-old and saw her 1- and 6-year-old children in the bathroom, police spokesman Paul Browne said. It's unclear how many times the children were stabbed.
The nanny was found with stab wounds to her neck, and a kitchen knife was nearby, police said. There was no water in the bathtub, they said.
Rima Starr, a music therapist who lives on the same floor as the family, said she heard screams coming from their apartment at around 5:30 p.m.
"There was some kind of screaming about, 'You slit her throat!' It was horrible," she said.
She said she believed the nanny had been hired just recently.
"I met her in the elevator, the day before yesterday, and was making small talk," she said.
After police arrived, she said, the mother remained in the building's lobby, screaming hysterically and clutching her surviving child.
The nanny was in police custody and was hospitalized in critical condition.
The apartment building sits in one of the city's most idyllic neighborhoods, a block from Central Park, near the Museum of Natural History and blocks from Lincoln Center for the Performing Arts. A drugstore is in the bottom of the building.