The mother of six-year-old Rikki Neave said she was telling the “truth and nothing but the truth” when denying his murder more than 25 years ago.
Rikki was strangled and posed naked in woods near his Peterborough home in November 1994.
His mother Ruth Neave was cleared of his murder the next year but convicted of child cruelty, the Old Bailey was told.
The killing remained unsolved for the next 20 years until a cold case review allegedly pointed to James Watson, who, jurors have been told, was seen with Rikki on the day of his disappearance.
Watson’s DNA was found on Rikki’s clothes, which were found discarded in a wheelie bin, the court has heard.
On Thursday, Ms Neave was called to give evidence in Watson’s murder trial.
Speaking via video link, she said she was living in a three-bedroom house in Redmile Walk with three of her four children at the time Rikki went missing.
Jurors were shown pictures of the inside of the home, including the bedroom Rikki had shared with his older sister before she went in to care.
Ms Neave told jurors that, by the time she was arrested on suspicion of Rikki’s murder in 1995, her other children were also in care.
Prosecutor John Price QC asked about her trial at Northampton Crown Court in the autumn of 1995.
He said: “When you were asked if you had murdered your son, what answer did you give?”
Ms Neave said: “’No, I did not.’”
Mr Price asked: “Was that the truth?”
Ms Neave replied: “Positively the truth and nothing but the truth.”
She went on to say she was jailed for seven years and released in 2000 after being convicted of child cruelty.
She told jurors: “I pleaded guilty because I was bullied in to it and I did not know what I was pleading guilty to.”
Ms Neave described calling 999 to report Rikki missing when he failed to return home from school on November 28 1994.
She said it was 12.03pm the next day when a police constable informed her Rikki’s body had been found.
Asked if she had been truthful to police, Ms Neave said: “I had no reason to lie. What is the reason to lie about your own child’s death?”
Ms Neave went on to describe events leading up to Rikki’s disappearance.
His step-father, Dean Neave, visited the house with friends the day before and promised to return to celebrate his and Ms Neave’s wedding anniversary, though he never did, she said.
She also recalled a disagreement with Rikki after he gave away some chocolates.
She told jurors: “I asked did he get me anything? He said, ‘There’s no reason why I would give you anything.’”
That night, Ms Neave said she slept badly due to her screaming baby daughter Sheradyn.
In the morning, Ms Neave woke Rikki at about 8.45am and told him to “get out of bed, get dressed”, she said.
She told jurors Rikki argued with his three-year-old sister Rochelle, with the pair “shouting and screaming”.
“He was fighting her, she was fighting him,” she said.
“It P’d me off – it was early in the morning.”
When Rikki went downstairs, he asked about breakfast and was told it was already on the table, then said hello to Sheradyn, Ms Neave said.
Mr Price asked: “Where were you when you last saw your son alive?”
Ms Neave said: “I was at home in my living room. I was sitting up, dozing, trying to keep my eyes open.
Mr Price said: “What was Rikki doing the last time you saw him alive?”
Ms Neave replied: “He was playing with Sheradyn. I was sitting there looking at them both, dozing.”
She said Rikki was wearing his school uniform, including a white shirt and grey trousers, and later identified them after they were recovered from the wheelie bin.
Ms Neave told jurors Rikki had a Game Boy he was fond of that she thought had been taken by police after his murder.
Jurors were shown images of items seized by police, including Game Boy headphones but not the handheld Nintendo console itself.
Ms Neave will be cross-examined on Tuesday.
Watson, now 40, of no fixed address, has denied Rikki’s murder.