Officers say Nelson admitted to spanking the boy for defecating on himself.
But Zion's older brother told investigators a more sinister story.
The five-year-old had old injuries of his own when he said his dad asked him to lie about the beating he heard.
"He heard a whipping sound, he heard Zion screaming, and then it stopped," said Ignacio Ruiz of the Fresno Police Department.
Just the description of the injuries Zion suffered brought Moneesha Camp to tears several times over the last four years.
Even with the case now drawing to a close, Camp could barely speak to the judge accepting her guilty plea.
BREAKING: Moneesha Camp is in tears as she admits to child endangerment in the death of 3-year-old Zion Nelson back in 2014.— Corin Hoggard (@corinhoggard) May 29, 2018
Her boyfriend, Donald Nelson, was Zion’s father.
He admitted to second degree murder five days ago.
Details coming up @ABC30 on Live at Five and at 6. pic.twitter.com/9EqEcIgeHb
Camp was Nelson's girlfriend and served as a mother figure to both of his sons.
Prosecutors initially charged her with the same crimes Nelson faced: murder, child abuse, and endangerment.
In the end, Nelson admitted to murdering Zion and abusing the older boy, and Camp admitted only to endangering Zion's life.
Camp's attorney Linden Lindahl said, "She was present when Zion was abused by his father, and she did not intervene."
But the attorney says she was too afraid, for herself and the boys, to stop the attacks.
"In the entire experience, I believe she was nearly as much a victim as the boys were. The only difference is she, unlike one of them, survived."
Nelson faces 19 years to life in prison when the judge punishes him in July.
Camp agreed to serve a six-year sentence, but she's already served enough time to get out, so by Wednesday morning, she'll be free to restart her life.