The boy's father has been charged for refusing a blood test after the accident.
The 39-year-old P-plate driver and his four children were in the car at Sheidow Park, in Adelaide's south, when the accident occurred.
Panicked residents heard the crash and ran from their homes to help.
"I heard the children screaming so loudly," one resident told 7NEWS.
The seven-year-old in the front seat was alive but badly injured and was raced to the Women's and Children's Hospital.
Witnesses say the boy's size potentially saved his life.
"It's extremely lucky that it wasn't an adult sitting in that seat because I don't think they'd be here," one witness said.
Three girls in the back seat, aged nine, 10 and 13, were treated for shock.
The father was taken to Flinders Medical Centre where police say he refused to submit to drug and alcohol testing.
He is now facing charges and will face court at a later date.
This story first appeared on 7NEWS and has been republished here with permission.