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.
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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?"
Watson, now 40, of no fixed address, has denied Rikki's murder.
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