Jordan Thompson's mother says she was told the sleeping toddler was fine and not to worry about him waking up in her absence, before he died while she took a trip to the shops.
More than 18 years later, Cecil Patrick Kennedy, 51, has pleaded not guilty to the toddler's manslaughter at Singleton in the NSW Hunter region on March 19, 2005.
Kennedy was looking after the 21-month-old and his own child while Jordan's mother, Bernice Swales, went to the shops with her daughter and Kennedy's two other children.
Jordan had been sleeping most of the afternoon and she wondered about waking him up beforehand and taking him with her to the shops, she told a court on Monday.
Kennedy reassured her.
"'He'll be fine, it's OK, don't worry about it,'" Ms Swales recalled being told.
She rushed the toddler to Singleton Base Hospital, across the street, after returning to the unit and learning he was unresponsive.
"I was running as fast as I could, flat out the whole way," she said in a video shown to the jury retracing her steps with police. She was holding a mannequin as she showed them how she had tripped on a garden bed and pointing out where a nurse had spotted her approaching.
Kennedy said something about the toddler having fallen either into or out of the bathtub, Ms Swales told the court, unable to remember specifically.
Jordan was unable to be revived and an autopsy did not identify a cause of death, but blood analysis detected high levels of an antidepressant Kennedy had been prescribed in the toddler's system, the NSW District Court jury heard.
Ms Swales said she met Kennedy after moving into the same unit block in 2004.
Their children would play together and the pair began a relationship a few months later, which became complicated about a month before the toddler's death.
"There was some speculation that Cecil was seeing another woman," she said.
"I was actually told by his sister as I had recently discovered I had fallen pregnant."
Crown prosecutor Kate Nightingale earlier told the jury she expected the other woman would give evidence about Kennedy calling her from a payphone talking about giving a child tablets to help them sleep.
Recordings from covert listening devices will be played to the jury, including a conversation between Kennedy and the toddler's mother after they were told an antidepressant had been detected in blood analysis, but not specifically what kind.
The woman is recorded saying she's never taken antidepressants and does not know what they look like, while Kennedy, prescribed the same substance found in the toddler's blood more than a year earlier, stayed silent, Ms Nightingale said.
Kennedy was also allegedly recorded talking to his mother about retrieving the tablets from his unit.
They were found by police in a chemist's paper bag on a high shelf in his cupboard, Ms Nightingale said.
Prosecutors seek to prove Kennedy killed the toddler by administering the antidepressant, or was criminally negligent leaving him unattended in the bathtub, knowing he was under the influence of a drug, or unwell.
Kennedy's barrister Linda McSpedden urged the jury to keep an open mind until the trial ends.
"A case of this nature can have the tendency to arouse emotions of sympathy and antipathy," she told the jury of eight women and seven men.
"Do your absolute best to focus on the evidence itself," she said.
The trial continues on Tuesday and is expected to last about eight weeks.
