The survey, commissioned by umbrella body Navca, revealed that funding cuts and increased bureaucracy had pushed many voluntary organisations out of local service delivery.
A total of 68 per cent of charities suffered reduced public sector funding in the previous 12 months and 51 per cent saw services they provided moved back in-house by the local authority or other public bodies.
In all, 79 per cent of charities said they did not believe the tender process was fair or transparent when they bid against in-house providers for contracts.
"It seems local spending decisions are going against the grain of government policy," said Kevin Curley, chief executive of Navca. "There is a danger we will lose the vital preventive work of small groups among the most disadvantaged communities. Government must act urgently to address the situation."
The survey was based on a poll of 173 voluntary organisations that provide a range of preventive services for children and young people. The children's sector has gone through a great deal of change, such as the emergence of children's trusts. However, many of the issues it has faced, including the switch from grants to contracts, are likely to be felt across the voluntary sector.
The survey found that charities were particularly affected by government-led initiatives such as Sure Start. The survey report reads: "The findings suggest very strongly that community-based provision is under threat. In some places the threat has been carried out."
A spokeswoman for the Local Government Association said changes in the way children's services were commissioned, the impact of "difficult" financial settlements and the Government's efficiency drive were all affecting charities.
"The challenge for local government and the sector over the next five years is to develop capacity with local voluntary and community projects so they can be supported to tender on a level playing field with other organisations," she said.
A spokeswoman for the Department for Education and Skills said local authorities should take all reasonable steps to encourage the involvement of all providers in the planning and delivery of children's services.
"It is up to local authorities how they commission services and use the funds available to them," she said.