The council says steps are already underway to address shortcomings identified in the report.
Following a two-week inspection, the council was rated “inadequate for safeguarding”.
Pete Bradbury, cabinet member for children and families, has announced the formation of a specialist team to speed-up the rate of progress.
“The report is deeply disappointing,” he said.
“We fully accept the findings and the criticism. We know what needs to be done and we are going to get on with it.
“I want to assure our residents the protection of children is an absolute priority as far as this council is concerned.”
Mr Bradbury said improvements had not led to the speed of progress the council would have liked since an inadequate rating two years ago and Ofsted had made it clear they needed to “step up the pace”.
He added the improvement team will be fully funded by the council’s contingency fund, so there will be no impact on council taxpayers or other services.
The team will report to an independently-chaired improvement board.
Of the 29 authorities inspected this year, 12 were rated as inadequate for safeguarding children.
The inspectors found West Sussex services for working with “looked after children” were adequate.
Mr Bradbury said: “Social workers have one of the most difficult jobs in society, and since becoming cabinet member for this service in May, I have been impressed with their hard work and dedication.
“What today’s report makes clear is that they need more support and better processes and procedures have to be properly embedded.”