Emergency services found the three-year-old boy at a home in Hackham West in Adelaide's south on September 19 after a member of the public raised the alarm.
He was covered in bruises, had burns on his limbs and his face, suffered significant abdominal injuries, eye injuries and spinal damage, South Australia Police Detective Chief Inspector Mark McEachern said.
The toddler was intubated for two weeks and has since been released from hospital but has a "long road of recovery ahead of him", according to Det McEachern.
"We're fortunate we're not looking at a homicide," he told reporters on Saturday.
"Significant injuries, when you see photographs of the little boy being intubated with lots of burns, lots of bite marks, it's really significant."
The boy's 18-month-old brother had what's believed to be a "minor" bite mark on his face but that injury is still under investigation, Det McEachern said.
Both boys have been placed into child protection.
An 18-year-old woman who police allege was babysitting the toddler on September 19 was arrested hours after the tip-off from the member of the public.
She has been in custody since being charged with criminal neglect and "aggravated cause serious harm with intent".
Det McEachern described her link to the three-year-old as being a family acquaintance.
"A number of these injuries are believed to have occurred prior to the babysitter taking custody of the child, we still believe that injuries were caused on the 19th as well but there's injuries that precede that date," he said.
Police believe someone who knows the boy was involved in the alleged abuse before that date.
"We're looking at family members, we're looking at anyone that may have gone to the house in the days in the leading up to the 19th of September, but it's clearly someone that knows the child well," Det McEachern said.
He said there had been "limited co-operation from people close to the little boy" during the investigation.
Anyone who visited the Hackham West home before September 20 has been urged to contact investigators.
The woman will face Christies Beach Magistrates Court on January 21, 2025.
Lifeline 13 11 14
Kids Helpline 1800 55 1800 (for people aged 5 to 25)
