Inspectors examined Nottinghamshire Police’s handling of six cases relating to children aged between 10 and 14 years who were at high risk of sexual exploitation.
One was assessed as “requiring improvement” while the other five were inadequate.
HM Inspectorate of Constabulary (HMIC) said it found “poor supervision” in all six cases.
It cited one example in which “long delays” occurred in an investigation into a 14-year-old girl’s sexual relationship with a 21-year-old man.
A frontline officer was given the task of completing an initial investigation before transferring the enquiry to the sexual exploitation investigation unit (SEIU).
The report said: “It took six months for this initial work to be completed, and a significant delay before the suspect was arrested.
“During this period, two further offences relating to the suspect, involving the exchange of indecent images with other teenage children, were reported to other forces.”
In four of the six cases, suspects were known to have access to other children who might have been potential victims but police failed to take action to manage the risk they posed, the watchdog said.
It went on: “For example, the delays in arrest noted in the case above meant that appropriate bail conditions, which might have prevented further offending, were not imposed on the suspect.”
HMIC raised concerns that non-specialist staff were investigating child protection cases without having received training in how to manage them effectively.
For example, in the case of a 10-year-old girl who had been groomed to send indecent images of herself over the internet, there was a 10-week delay in requesting analysis of the suspect’s computer “because the officer did not understand the procedure”.
The report said: “During this time the suspect closed his Facebook account and potential evidence was lost.”
The inquiry is part of a rolling programme of child protection inspections of all forces in England and Wales.
HM Inspector of Constabulary Zoe Billingham said: “We were concerned to find that non-specialist staff were investigating cases without receiving the training they need to manage these effectively.
“We also found examples of poor supervision of investigations leading to unacceptable delays, which can leave opportunities for perpetrators to continue offending.”
In four cases the initial response was prompt, the report said, but there was a five-day delay in responding to a report of the sexual grooming of a 13-year-old boy and a 10-day delay in the case of a report concerning a 14-year-old girl in a sexual relationship with a 21-year-old man.
When officers did respond, they took immediate action to ensure children were safe.
Ms Billingham said that since an initial probe in September 2014, Nottinghamshire Police has “made a clear commitment to improving the services it provides to children”.
She added: “In particular, it has improved procedures for considering children’s welfare when attending domestic abuse incidents, and improved the supervision of missing children cases.”
Referring to the case involving a 14-year-old girl discussed in the report, the force said: “The length of time taken in this investigation actually demonstrates positive work by Nottinghamshire Police.”
It added: “When a victim would not engage with us we did not give up and continued to pursue the offender over a series of weeks until we could build up enough evidence to arrest him.”
The force said there were some cases where “it is completely appropriate for non-specialist staff to deal with offences which relate to child protection matters, with the help and advice of specialists”.
Chief Constable Chris Eyre said: “The safety and welfare of children is paramount, and at the heart of everything we do.
“I’d like to reassure people that we have expert officers and staff working extremely hard to keep young people safe from harm.
“Indeed our good progress in domestic abuse cases involving children has been highlighted in the report.”
HMIC said it was pleased to find the force has taken steps to ensure children were not detained in custody under section 136 of the Mental Health Act.
Mr Eyre added: “We are also particularly proud that since April 2015 no child has been detained in police custody under the Mental Health Act.”