Emilia Thomson, the toddler knocked over by her mother in a Bibra Lake driveway, was "the light and joy" of her parents, according to the family priest.
Father Bryan Rosling, who baptised Emilia in October, said Gabriela Thomson and her husband Chris were devastated by the loss of their third and youngest daughter.
Emilia died in Fremantle Hospital on Sunday after being hit as Mrs Thomson reversed the family's Saab station wagon out of their driveway.
The 16-month-old's sisters Anna and Rafaela are believed to have been in the back seat.
Major crash detectives are investigating how Emilia came to be in the path of the car.
Father Rosling, who has been in regular contact with the family, spoke of how cherished Emilia had been.
"They treasured her," he said.
"She was a happy little child, always smiling -a typical little 16-month-old running around."
Father Rosling said a "fragile" Mrs Thomson had no recollection of the incident.
"It is just the most horrible thing, you can't explain it," he said.
"Kids that age are so quick, so invisible."
A neighbour, who did not wish to be named, said Mrs Thomson and her husband were "beautiful, quiet and happy" people.
She said "Gabi" was the most caring mother she knew.
"Her whole life is her children and her family," she said. "It's not fair. They're a lovely couple."
Flowers and cards sat uncollected at the front door of the Thatched Court home yesterday.
The family have been staying at Mr Thomson's parents' home. Mr Thomson made a dash from the US to be with his family and was due back yesterday.
Father Rosling said Mrs Thomson was from South America and had few family members in Australia. A registered migration agent, she volunteers as a catechist at Mater Christi Catholic parish in Yangebup and she and her children were going to Mass when the incident happened.
"I can't imagine a family who deserved this less," Father Rosling said. "They are beautiful, beautiful people."