Wiltshire Police has been told it needs to improve how it safeguards and protects children in a report by Her Majesty's Inspectorate.
The report found the force's response to missing children was lacking, and staff did not have the right training for incidents involving children.
Inspector of Constabulary, Wendy Williams, said she will monitor the force to make sure it improves.
Wiltshire Police said it accepted the recommendations in the report.
It has six weeks to set out how it will respond to the issues raised, and it will be inspected again in six months.
The report, which was compiled in March 2022, said the force needs to make "fundamental changes."
"The inexperience of many of its officers means they don't have all the skills needed to respond to, assess and investigate all aspects of child protection," it said.
While the force's officers who deal with child abuse investigations were praised, the report said frontline staff didn't have enough understanding of good child protection practice.
In one case study, the mother of a child reported that a registered sex offender was approaching children and offered to build one a tree house.
Officers delayed interviewing the offender for nearly a month and it took two months after the incident before the offender was issued with a warning letter for breaching a sexual harm prevention order.
'Not good enough'
"Throughout the force, officers do not always understand the importance of speaking to children, listening to them.
"The force's response to missing children isn't good enough," said Ms Williams.
Assistant Chief Constable Deb Smith said: "We take our responsibility to safeguard those most vulnerable incredibly seriously.
"We have taken prompt and comprehensive action to address the issues raised in the report.
"We've also focused on delivering enhanced training for our frontline officers."