The Child Exploitation and Online Protection Centre, a government intelligence and enforcement agency reporting to the Home Office, is urging primary teachers to give more advice to their pupils about internet dangers.
The centre believes that online predators are increasingly turning their attention to primary pupils. Parents are reporting around 10 such incidents every month to its website. A cybercafe is being launched next week to provide advice to children aged eight to 11.
It encourages younger children to first talk to an adult they trust, such as a teacher, and the site's "virtual police station" can put them in touch with a real police officer.
The centre's concerns had been mainly about teenagers. But Jim Gamble, chief executive, said reports of abuse from younger children was causing concern.
Mr Gamble urged teachers to take children to the cybercafe and to use its lesson plans and resources. Helen Penn, the centre's education head, said she had been astonished at younger children's technical sophistication. They were using eBay accounts, accessing the internet by mobile phone, and chatting to people around the world via interactive games such as World of Warcraft.
"They don't have the same instincts as an older child," she said. "They don't have the same gut feeling of caution that they would if they were approached by a 40-year-old in the park."
A study for the South-West Grid for Learning has found that primary schools, as well as secondaries, have problems with social networking sites.
Karl Hopwood, head of Semley CE Primary School in Shaftesbury, Dorset, helped the centre and Becta, the Government's education technology agency, to develop the teaching resources. Some of his pupils had naively posted their addresses on Bebo and he knew of other primary children in the South West who had face-to-face encounters with adults they had met originally online.
"For a lot of parents and teachers, they think the worst thing that can happen is children accessing inappropriate images," he said.
"When I point out what their children are doing, the colour drains from their faces."
Britain's first social networking site for primary children is to launch later this year - with safety from bullying and online predators its number one focus. Gridclub Max, a subscription site, is intended to wean children away from riskier adult sites.