A police source told the New York Daily News the mother of the children -- a 5-year-old boy and his infant sister -- had left a suicide note and confession saying she intended to kill her children and herself.
The two children were found unconscious in a Bronx apartment before midnight Thursday and were later pronounced dead, the New York Post reported.
A 29-year-old woman found inside the apartment with lacerations to her wrist was taken to Jacobi Hospital, where she was in stable condition.
The Post said an off-duty police officer who lives in the apartment discovered the mother and children.
The relationship between the officer and the victims remained unclear, the Post said.
The identifications of the victims had not been released.