Kayley Ishii was in the front-loading washing machine for at least two minutes before her mother found her on Monday afternoon, the Los Angeles Times said.
The machine was easily started by the boy as the on/off button was about 20 inches (50 cm) from the ground.
"The mother frantically tried to rescue the daughter and called paramedics," the sheriff's spokesman Jim Amormino told the newspaper.
The child was rushed into a hospital but died five hours later, he said.
Doctors said the girl died of blunt force trauma. Investigators determined that the death was "accidental."