The owner of the dog responsible for the attack left the scene without giving her details.
Now the Stein family is urging her to come forward with an apology.
Putting on a brave face, 10-year-old Hollie Stein is missing her best mate.
Charlie the Jack Russel was taken to the vet on Friday but never came home.
"That dog killed my best friend," the grieving girl told 7 News.
Hollie was walking Charlie at Kingsbridge Park about 5:30pm Thursday when a larger black dog attacked.
"He ran out of nowhere and started biting Charlie and picking him up and everything."
Hollie said the dog's owner, a woman, was sitting nearby and she had to scream to get her attention.
"She said 'I'll pay for the vet bills give me your mum's number.'
"And I was worried about Charlie and I kept saying just follow me home it's right across the road and she was like 'no give me your address' so I gave her our address.
But to Hollie's disappointment the woman never showed and Charlie's condition deteriorated overnight.
A visit to the vet the next morning revealed he had a ruptured kidney and severed pancreas.
I have a problem with this. This little dog is mauled by a larger dog and they don't bother to take him to the vet? It says it happened about 530pm. Yes, most vet clinics close around that time, but why didn't they take him to an emergency clinic?
Obviously they live close to the park. Google Maps says Joondalup Central Veterinary Hospital is just a 20-minute drive away. They were open until 7pm.
The EVH Emergency Vet Hospital is a 25-minute drive away and it says it's open 24 hours/day.
I'm sorry for this little girl, but I'm more sorry for Charlie who suffered in agony for more than 12 hours before her parents finally took him to the vet who ended his suffering.
Hollie's mother Nat Stein said they wanted the woman to come forward and apologise.
"We're not after money we just want you to come say sorry to Hollie just to give her some closure on what's happened and so I can show Hollie what the right thing is to do," she said.
Nat Stein wants to talk about doing the right thing? Doing the right thing would have been for YOU to rush little Charlie to the vet immediately after your crying daughter carried him home and told you he had been mauled by a dog. I'm adding an "animal cruelty" tag for this because I'm so angry that she's being all moral about this dog owner 'doing the right thing' when she didn't.
Hollie said the dog's owner, described her as being Caucasian, was wearing a white singlet with a skull printed on it.
The Stein family says the City of Waneroo ranger is investigating, checking nearby CCTV for clues.
Anyone with information is encouraged to call Crime Stoppers 1800 333 000 or use the Crime Stoppers online reporting page.
(Yahoo news - May 20, 2017)