The survey, carried out in five areas of England and Wales, found there were places available for fewer than one in six children on organised holiday play schemes.
In a report published last week, before the murders of three children, the charity calls for more affordable, high quality schemes to enable children to play in safety, under supervision from adults.
All the areas in the survey - mainly housing estates in Birmingham, Cardiff, Sunderland, Rochdale, and South London - have many low income families.
Yet 85 per cent have no hope of places, the charity says. Extrapolated across the country, it could mean as many as 1.2 million children with no place to play under adult supervision this summer.
The report's author, Paula Keaveney, said: "The situation is grim, but the future looks worse. Most of the local authorities interviewed said the prospects of children finding places on summer schemes were diminishing. "
The murder of two schoolboys while out fishing on Merseyside last weekend, and the abduction and murder of a seven-year-old girl in north Wales, has highlighted the fears of parents over allowing their children to play unsupervised.
A recent Policy Studies Institute report found that just 9 per cent of seven and eight-year-olds went to school on their own, compared with 80 per cent in 1981.
The National Voluntary Council for Children's Play, an umbrella group representing children's organisations, is launching a campaign urging local authorities to provide more play opportunities for children by tackling traffic hazards and providing more supervision in public parks.
Meanwhile, the three main local authority associations have complained to the Department of National Heritage over its plans to put out to tender the Government-funded National Play Information Centre, which collects and provides information on children's safety and oversees education and training of play supervisors.