Covina police Lt. John Curley says 28-year-old Alex Trujillo has been treated for self-inflicted wounds and booked on suspicion of murder.
According to Curley, the attack on the 2-year-old happened in a Covina home Tuesday night, and Trujillo was seen stabbing himself after responding officers surrounded the house.
Trujillo was restrained after officers kicked open a door.
Curley says the boy was taken to an area hospital, where he died. The boy's name had not been released.
Covina is about 20 miles east from Los Angeles.