Hui Zhang, 34, had admitted killing the couple, Ying Wang and Liangsi Xui, during a violent row at her home after they learned the little boy had died.
The judge followed the prosecutor's recommendation of 20 years behind bars for Hui, who had claimed she acted in self-defence as the furious parents attacked her and her boyfriend Te Lu, also 34, with a butcher's knife.
Te, who was a co-defendant in the case, was acquitted.
As the trial opened on Tuesday, Hui confessed to the killing, saying: “It's true, I killed them, and I will regret it for the rest of my life.”
She and Te had decided to offer the child's parents money to try to get them not to report the boy's death. They invited the parents to their home, but said their plans quickly went awry faced with the fury of the grieving couple.
Te testified that he fell unconscious during the fight and remained so while Hui chopped up the two bodies in the bathroom with an electric saw, using the washing machine to cover the noise.
“I was sucked into a whirlwind of nightmares but I am innocent,” Te told the court.
Hui then wrapped the body parts in rubbish bags and scrubbed her apartment clean. Te later helped her get rid of the remains, transporting them “by foot or public transport” to the forest of Vincennes, east of Paris, a policeman said.
Prosecutors cast doubt on Hui's claim of self-defence and said there was no proof Te had taken part in the murders.
The case first came to light in June 2012 after two joggers came upon a leg, cut off at the ankle, in Vincennes forest. Several days later, a guide dog found a human torso in the same area, but the hunt for further remains was fruitless.
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Before the bodies could be identified, Hui and Te turned themselves in. They then directed police to the locations of more body parts around the forest.
However, they did not find the baby's body, which Hui said she had thrown in rubbish bins along with some of the other remains.