In one case, police failed to investigate men who had allegedly given drugs to a girl in exchange for sex
Children in care homes were left vulnerable to abuse because of "shortcomings" by South Yorkshire Police, a report has found.
The HMIC investigation was carried out in May - just months before a report found 1,400 children had been sexually exploited in Rotherham.
It also criticised South Yorkshire Police for "unnecessarily" holding children in custody overnight.
But inspectors praised recent progress made by the force.
HMIC said that in some cases, officers interviewed children sensitively, pursued evidence from a range of sources and made detailed searches of homes and computers.
However, inspectors were concerned by an incident where police and social workers dismissed possible sexual injuries to a four-year-old girl as eczema.
In another case, police failed to investigate men who had allegedly given drugs to a girl in exchange for sex.
'More must be done'
HMIC found that while some investigations were thorough, other cases of missing children resulted in little police activity and no search for suspects.
"We are concerned that force practice is inconsistent so not all children receive the standard of treatment they deserve," said Dru Sharpling, of the HM Inspector of Constabulary.
Children in care were found to be particularly vulnerable. One 13-year-old girl was found, with condoms, at the home of a sex offender.
There was no record of social services or her parents being notified, or of any inquiries about the man whose home she was found, the report said.
Children in care were sometimes found to have breached bail and were placed in custody, often "unnecessarily", investigators found.
Mr Sharpling said: "More must be done to improve the care of children in custody."
The force was "clearly prioritising child protection" and had "made some good progress", he added.
South Yorkshire Police welcomed the recommendations of the inspection and said inconsistencies highlighted by the report regarding the force's response to child sexual exploitation were being addressed.