Gwent Police has been told to immediately review its policy on keeping children in custody after concerns were raised by inspectors.
Her Majesty's Inspectorate of Constabulary and Fire & Rescue Services (HMICFRS) raised the matter after discovering children were typically being held in cells in the same area as adult offenders - sometimes for hours while awaiting a representative.
In one case, a 15-year-old boy arrested for robbery was strip-searched without an appropriate adult being present. The search was not urgent and no-one arranged for the appropriate adult to come to the police station for seven hours, a report said.
The force was told to immediately review its policy on keeping children in custody, making sure they are detained only when necessary and for the absolute minimum amount of time as well as seeing that appropriate adults attend the police station promptly.
HMICFRS is inspecting the child protection work of every police force in England and Wales.
While reviewing the Welsh force, inspectors also found officers failing to properly investigate when registered sex offenders were breaching their conditions by moving addresses and not notifying police as the law requires.
Some of the offences were not investigated or recorded. When the breach was treated as a crime, the offender was not arrested, according to inspectors.
In one case, police failed to arrest a sex offender found living with a woman and children aged two and five, which was in breach of his court order, and no further action was initially taken.
He was twice more found at the address by officers on subsequent visits. He was not arrested but was later prosecuted.
The force was told to improve its procedures in this area within three months.
Another inspection will be carried out again within six months.