Penny Lloyd, professional officer for the British Association of Social Workers Wales, said the failings identified in the report from the principality's social services inspectorate could only be addressed if the Assembly government provided more money and more staff were recruited.
In its report on an audit of local authority services following the publication of the Victoria Climbie Inquiry report in January 2003, the inspectorate said services for children in need in Wales were not consistently good enough.
It identified problems in areas such as the management of referrals, accessing services, quality assurance and record keeping. However, it did identify "many examples of good and innovative practice".
The report, which was presented to the Welsh Assembly's Health and Social Services Committee last week, said all local authorities in Wales had experienced a significant increase in the demand for children's services and a major rise in the number of referrals.
Lloyd said this increase could be due to a "widening gap between the haves and the have-nots", while Joe Howsam, spokesman on children's care for the Association of Directors of Social Services Wales, said heightened awareness among the public could also have led to the increase.
The report also said some councils were operating "very high threshold criteria" for people to access services.
"Most councils seems to be operating an absolute crisis service," said Lloyd. "I would imagine this means that most families don't come to the notice of a social worker who's prepared to do anything until things have got really bad. There's a dearth of investment in preventative services across Wales."
Lloyd said BASW members were also complaining about lack of supervision and training, another point highlighted by the report.
Howsam said most authorities had enough money to recruit more managers and frontline workers, but there was a problem in recruiting the right staff.
"There's been a big thrust to make social work in Wales more attractive and we want more people to come into the job, but it's difficult," he said.
Graham Williams, the chief social services inspector for Wales, said: "Authorities must learn from those who are providing a good service to vulnerable children and their families."
The report was based on self-audits by all local authorities in Wales.
They had been issued with an audit framework and electronic tool by Williams that covered the social care recommendations made by Lord Laming in his report into Victoria Climbie's death. Inspectors also paid two-day visits to councils earlier this year.
