The boy was found dead at home in Kirwan, Townsville about 7pm on Saturday and the woman has been charged with his murder.
Queensland Police's Detective Senior Sergeant Dave Miles said the two had a "loose" family connection and the boy was in her care at the time.
He said the child suffered a "significant head injury" and described the scene as "confronting".
"The injury is consistent with what I would say at this stage appears to be blunt force trauma," he told reporters on Monday.
"We will await the outcome of the autopsy before we make an absolute determination what weapon may have been used."
The detective said police had focused their investigation on a certain period of time on Saturday.
"We believe the child may have sustained the injury within eight hours, as to what point the child was deceased will be a matter for the autopsy," he said.
Det Sen Sgt Miles revealed other children were at home at the time but no other adults.
"Anything involving a traumatic injury and the death of a child, which in circumstances like this occurs, is quite abhorrent," he said.
"No child should be subjected to anything that obviously leads to this sort of outcome for them."
The 24-year-old's case was briefly mentioned in Townsville Magistrates Court on Monday.
23°C