Carmen Iacullo, 33, allegedly tortured the boy earlier this month at the Placentia home of his girlfriend, Lori Pincus.
Iacullo is accused of beating the boy, stabbing him with scissors, pinching him with pliers and burning him with cigarettes and a lighter.
"You do not have to know the legal definition of torture to take one look at these pictures and know this child was tortured," said Det. Corrine Loomis of the Placentia Police Department."He burned him with an open flame. Picked him up and dropped him by his genitals."
Police say Iacullo moved in with Pincus three months ago. The alleged torture took place over a few days.
The boy's mother is suspected of knowing about the abuse but not calling police and not seeking medical help for her son.
"She might have had that child, but she's not a mom," said Pincus' neighbor, who identified herself only as Christina.
Christina said she saw the boy playing outside a week ago and wishes she would have looked more closely.
"I was shocked, saddened and I feel sorry for the little boy," she said.
Police said Iacullo was arrested June 8 on an unrelated parole violation.
Police said Pincus left her child home alone a day after the arrest to visit Iacullo in jail. When the boy wandered outside to a nearby gas station, witnesses called police.
Authorities said the boy is in protective custody.
Iacullo is being held on $1 million bail and Pincus is being held on $100,000 bail.