The couple have had three children in their 17-year relationship, and the 12-year-old is the first child to give evidence in the trial, which began Tuesday.
He was aged 10 at the time of the incident - October 27, 2013. His sister was 12 and his brother 13.
Ms Pereniko told Crown prosecutor Harry Mallalieu she confessed to cheating and thought it would be the end of the relationship. The pair had been drinking at their home with Ms Pereniko's sister Lynette Pereniko, who went to bed before the incident.
The complainant described seeing Kuru holding the knife, and then the knife being in her chest.
"I didn't even realise it had happened," she said.
During cross-examination she admitted there had been times in the past when she had attacked Kuru, but denied ever using weapons.
Lynette Pereniko described how Kuru took her cellphone from her as she tried to call for an ambulance, saying: "Don't ring the police, sis."
Mr Ross said Kuru had taken the phone, but did it because he wanted her to drive them to the hospital, rather than wait for an ambulance.
The next witness was Dr Alexander Skavysh, the general surgeon at Whanganui Hospital who tended to Ms Pereniko's wound. He said the wound was about 2cm long, 3-4mm wide, and "at least" 3-4cm deep.
"It was quite forceful, enough to penetrate all layers of the chest wall and injure the lung," he said.
"The patient couldn't push herself on this knife; somebody created this injury to her body."
The final witness for the day was the 12-year-old son of Ms Pereniko and Kuru. In a video statement made to a CYFS social worker, he said he was asleep in his room when he woke to hear his parents fighting.
He went to see what was happening, and saw his father trying to kill himself by cutting his wrists with the knife, he said.
"My mum said she was in love with someone else and my dad said he was going to kill himself."
When his mother was stabbed accidentally, she told gathered family members Kuru was "going to kill all of us".
The trial continues today in front of Judge David Cameron and a jury of seven men and four women. One female juror was excused yesterday morning because she was unwell.