The four-month-old baby was in a stroller when his 21-month-old cousin pushed it, causing it to fold and fall to the ground.
After the fall, the baby did not cry and appeared to be fine. But he was found to be unresponsive the next morning.
A frantic effort by his aunt and paramedics to save him was unsuccessful. He was later pronounced dead.
In a coroner's inquiry into his death yesterday, the court heard that he had been under the care of his aunt, 29, the mother of the 21-month-old, when the tragedy occurred. She is the half-sister of the baby's 33-year-old mother.
The deceased cannot be named to protect his young cousin's identity.
On March 2 last year, his aunt, Ms Linda, and her fiance, Mr Zul (not their real names), 22, decided to get tattoos.
The jobless couple took both children in separate strollers to Nee Soon Tattoo Shop at around 3pm that day. They took turns looking after the children as the other got inked.
Investigation officer Staff Sergeant Eugene Chua told State Coroner Janet Wang that the couple took a taxi back to the baby's mother's flat at about 8pm.
At around 10pm, Ms Linda wanted to make a sandwich and the couple decided to head to a nearby supermarket with the two children.
The baby was placed in his stroller on the corridor and Mr Zul went back into the flat to get the toddler's stroller.
It was then that the toddler pushed the stroller, causing it to fold up and topple to the ground with the baby inside.
Staff Sgt Chua said: "According to (the baby's aunt), the stroller folded up because it was not fully unfolded and the safety lock was not properly secured.
"(She) immediately carried (the baby) out from the stroller. (She) observed that (he) was not crying and appeared to be fine."
She could not tell if the baby had hit his head against the frame of the stroller or the floor when it collapsed.
After that, she unfolded the stroller completely until she heard a click, which meant that the safety catch was locked.
All four then went to the supermarket and returned at about 11pm, after which Ms Linda put the children to sleep.
She and Mr Zul fell asleep at about 1am.
At around 6am the next day, Ms Linda heard her nephew crying and got up to make some milk.
But he did not want to drink it and he fell asleep after she gave him a pacifier.