HM Inspectorate of Constabulary (HMIC) found heavy work loads meant staff in child abuse investigation teams were unable to manage their investigations effectively.
Inspectors also found that officers did not always understand when to refer child protection issues to other agencies and found that children were being unnecessarily detained in police custody overnight.
The handling of five out of nine cases of child sexual exploitation which were examined by inspectors was assessed as inadequate.
The force was told it needed to take 'immediate action' after finding the attitudes of some officers towards victims of child sexual exploitation were 'unacceptable'.
Inspectors found nearly half of the responses to 115 cases involving children in June were inadequate.
Some sexual abuse cases faced 'unnecessary and long delays'.
Nine out of 11 cases involving children missing from home were not handled properly, while some officers were 'frustrated' by care homes reporting youngsters as missing because they were passing responsibility to police, the inspectors said.
In one child sex exploitation case, information was sent to the force's intelligence unit about men who may have been raping younger girls but no follow-up action took place, the report said.
They also raised concerns about delays in the arrests of offenders, and found officers had closed cases without sufficient consideration of the risk posed to other potential victims, the report found.
The report concluded: "Overall, the force's response to tackling child sexual exploitation has been slow, with inconsistent practice across the force area. There was a general lack of understanding of the extent of exploitation.
"Some of the attitudes officers held towards potential victims of child sexual exploitation or children who ran away were unacceptable and resulted in poor decision making.
"Staff need to understand that children do not make a 'lifestyle choice' to be abused, particularly those who are more vulnerable because of the neglect they have already suffered in their life.
"We recommend that West Midlands Police takes immediate action to review its plans for identifying, disrupting and prosecuting perpetrators involved in child sexual exploitation."
HM Inspector of Constabulary Dru Sharpling added: "The force needs to improve both its approach to the more complex child protection cases and a better understanding of the extent of child sexual exploitation in the West Midlands.
"I would like to encourage West Midlands Police to address our concerns immediately, and have asked that within six weeks it provides us with an action plan to demonstrate how it will act upon these recommendations."
West Midlands Police Assistant Chief Constable Carl Foulkes said: "I want West Midlands Police to be the best in country - dealing with vulnerable children with professionalism and compassion.
"Our Public Protection Unit has been doubled in strength to some 800 officers and staff. That means around 10% of the entire force are engaged in the fight. That displays our level of commitment.
"It's unfortunate that the HMIC inspection came just two days into new arrangements so do not reflect our exciting changes as there was so little time for them to be place.
"We cannot do it alone. We must work hand in hand with our partners, especially the local authorities.
Crime prevention minister Norman Baker said: "I am very concerned by the contents of this HMIC report. The safety of our children, and the elimination of child abuse, are top priorities for this department and the coalition as a whole."