The death of a baby boy found inside a car parked near a Perth childcare centre yesterday is a tragic mistake, police say.

The boy, believed to be 11 months old, was found inside the back of his father's car outside the Ladybugs Early Learning and Care Centre in Helena Valley yesterday afternoon.

His father arrived at the centre to pick up his son, but was told by staff he had not dropped him off that morning.

The boy was then found still strapped inside a capsule in the car.

Paramedics tried to revive the boy but were unsuccessful.

The incident happened on Perth's hottest day in the past month.

Police spokesman Sam Dinnison says the death is not being treated as suspicious.

"Staff from the daycare centre immediately commenced CPR, however their attempts to revive the child were unsuccessful," he said.

"At this time the child's death is not being treated as a homicide, but inquiries into the circumstances leading to the child's death are continuing."

The general manager of the childcare centre, Kim Beange, said staff administered CPR as soon as they were alerted.

"At no point were other children in attendance aware of the circumstances occurring," she said.

"All staff have been offered professional counselling and support services."