Police say the 18-month-old is still alive but received a brain injury and some fractures.
Police originally responded to a home in a Visalia neighborhood on Sunday for a report of an unresponsive child.
The child's foster mother, 29-year-old Darlene Valdovinos-Aguilera, told police the 18-month-old had received the injuries from a fall.
The child was brought to Kaweah Delta Medical Center and then transferred to Valley Children's Hospital.
Visalia Police say their detectives then worked with the hospital's child abuse team and determined the child's injuries were not accidental, but inflicted by Valdovinos-Aguilera.
"I think any case involving a child is difficult," said Visalia Police Sgt. Gary Williams. "But we make sure that we get all of the facts, we obtain all of the evidence that's out there, and then we make a decision based on that on whether there's an arrest that can be made."
After an interview with Valdovinos-Aguilera on Friday morning, police arrested her.
They say there was also a foster father living at the home, but he is not under investigation for any crimes at this time.
Child Welfare Services has taken the other children that lived at the home into their care, but police don't believe they were abused.
"We talked to everyone in the residence including the other kids," Williams said. "And so everyone cooperated, along with the mom."
The suspect is being held at the Tulare County Pre-Trial Facility on $500,000 bail.