Changes will be made to how youngsters are dealt with after they go missing in a bid to tackle the problem, Beate Wagner said.
Inspectors' latest findings have yet to be published, but Ms Wagner said that Ofsted found significant progress in a number of areas.
But she said the authority knew improvement was needed in other parts of the service.
To deal with the risk of exploitation, more staff will be recruited and return home interviews, which are conducted when missing children in care are found again, will be allocated to a single team of social workers.
Speaking about the Ofsted visit at a children and young people scrutiny committee meeting on Wednesday, Ms Wagner said: "Most importantly of all, we need to do more to strengthen our understanding and support for children who go missing from home and who are at risk of exploitation.
"Our current systems for talking to children when they return home after they go missing are quite complicated.
"Some children are seen by an external provider, some are seen by our staff and others are seen by the police.
"That doesn't really give us an understanding of what's coming out of all of those interviews."
Ms Wagner said that ensuring all interviews were carried out by one team would help "engage" troubled youngsters and convince them that it was worth talking openly about their issues.
She added: "We are going to get significantly more staff into that team, to make sure that young people who are at risk of sexual exploitation or drug and gang exploitation will be supported."
Councillors were also told that oversight of children's health needed to improve, amid reports some youngsters are missing important trips to the dentist and doctors.
Ms Wagner said children in care were now getting better access to mental health services than before, but waiting times still need to be brought down.
Areas of good practice highlighted by Ofsted include the leadership of the service, the rising number of visits made by social workers to youngsters, and the falling number of caseloads they are having to take on.
Staff now have an average of 20 cases each, down on the 30 plus they were dealing with when Ofsted placed children's services in special measures last year.
Ms Wagner said: "When you think of where we were before, that's a significant improvement.
"That's really been because we were able to get extra resources and capacity in quickly to deal with cases and that made sure caseloads were reasonable."