The victims, ages 2 to 29, were severely malnourished, suffering from muscle wasting and stunted growth.
At a news conference before the proceeding, Riverside County District Attorney Mike Hestrin described what he said was a case of “human depravity.”
The children were denied food, basic hygiene and medical care and were punished for perceived infractions such as washing their hands above the wrist.
“The parents would apparently buy food for themselves and not allow the children to eat it,” he said.
“They would buy food, including pies, apple pies, pumpkin pies, leave it on the counter, let the children look at it but not eat the food.”
As a result of malnourishment, the 12-year-old child was the weight of an average 7-year-old while the oldest, a 29-year-old woman, weighed just 82 pounds, he said.
Billy Baldwin and his mother bought the Turpins former house in Texas a year after the family moved out.
Inside, he claims it was “waist-deep in filth”, with “dead dogs and cats in there” and the smell “rancid.”
Faeces was littered across the living room, which appeared to have been used as a makeshift classroom, while the rooms had no beds – only mattresses on the filthy floor.
But that was not the strangest thing he noticed, he told the LA Times. He said: “Everything had locks on it: The closet had locks, the toy chest, the refrigerator.”
He added the family’s Ford F-150 truck was also piled high with filth and waste – stuffed full of dirty nappies.
And he claimed the whole property was strewn with empty Vienna sausage cans, adding: “It seemed like that’s all they ate.”
David Turpin, 57, and Louise Anna, 49, faces 94 years to life in prison if convicted on more than two dozen charges including torture, child abuse and false imprisonment
The father, is also accused of sexually abusing one of his young daughters.
The couple, who were married in 1984, pleaded not guilty to all the charges when they appeared in court on Thursday.
The children, including seven adults, are being cared for at local hospitals after being rescued by police.
A criminal complaint has revealed that all 13 of their children have names starting with the letter J, with three named by a childhood friend as Jennifer, Jessica and Josh.