Police from Holroyd Local Area Command performed CPR on the girl before she was rushed to The Children's Hospital at Westmead in a critical condition.
The father and other family members from the household called emergency services.
A neighbour, who did not want to be identified, said she was having a cup of tea with a friend on her front porch when the mother appeared at the front of the house.
"She [the mother] said, 'Your son hit me, your son has killed my daughter'," the neighbour said.
"I've never seen anything like it. I thought it was a big fire and that fire engines were going to show up but suddenly the street was full of cops."
Police say the child was repeatedly beaten at a property in Marian St, Guildford, before the girl's mother was able to intervene and jumped in the car, driving to McCredie Rd.
Police established crime scenes at both addresses and forensic units were called into examine the properties last night.
Neighbours said the young couple had only recently moved into the area, kept to themselves and were living in a granny flat at the rear of the large Marian St home.
Police were unable to confirm the exact nature of the girl's injuries last night but were interviewing the 22-year-old man at Merrylands Police Station.
