Bernice Swales says she warned Cecil Patrick Kennedy to move his medication out of her children's reach, months before her toddler would die under his care.
Kennedy was looking after the 21-month-old Jordan Thompson and his own child while Ms Swales, went to the shops with her daughter and Kennedy's two other children on March 19, 2005.
When Ms Swales returned an hour later, she saw Kennedy giving CPR to the unresponsive child.
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.
More than 18 years later, 51-year-old Kennedy is facing trial after he pleaded not guilty to Jordan's manslaughter at Singleton in the NSW Hunter region.
Ms Swales said she had seen a chemist bag on the accused's kitchen bench in the months before Jordan's death and asked him to move it on at least one occasion.
"I said, 'would you be able to put it up on the kitchen cupboard'?" she told Sydney's Downing Centre District Court on Thursday.
"He said that children don't touch it."
The day Jordan died, Ms Swales screamed at Kennedy to call an ambulance before whisking the toddler into her arms and running to the nearby hospital.
Doctors spent an hour giving him adrenaline and trying to revive him to no avail.
Ms Swales' sister Nicole told the court Kennedy explained what had happened while the family waited.
"Jordan had been asleep then he (Kennedy) put him in the bath and went to the PlayStation," she recalled him saying.
Kennedy's son had called to him from another room while playing video games.
When the 51-year-old returned, he found the toddler facedown in the water.
The accused previously told police in an interview that he had been gone for 40 seconds.
Ms Swales later that day asked Nicole to help check on her other children at home where she recalled the toddler's older sister said: "Mummy's gone to hospital with Jordy because he's slipped in the bath and bumped his head."
Jordan's mother 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.
The trial continues.
