Many children aged nine to 11 are indulging in very risky behaviour online, suggests a survey.
Many are sharing personal information and playing games rated for much older children, found the survey drawn up by the ISC2 IT security education group.
In addition, 18% of the 1,162 children queried said they had arranged offline meetings with friends made via the web.
Meanwhile, a second survey suggests 55% of young people in England accept cyberbullying as part of everyday life.
Security experts have urged parents to help their offspring stay safe by being more involved with what they do online.
"[Many] primary school children know far more about the internet than their parents do," said Tim Wilson, an information security worker who carried out the survey for the ISC2 organisation which helps schools educate children about web safety. In total, 15 schools in south-east London, Kent and Guernsey took part in the survey.
"Youngsters actually participate in a lot of risky behaviours that I don't believe the parents know about," he said. "Not because of any ill will on behalf of the parents, it's just that the parents do not understand technology."
The survey exposed the amount of time that younger children were spending online and what they got up to.
It found that 43% went online every day and 46% spent more than two hours each time they browsed. Many (22%) regularly used it after 21:00 and a small percentage, 7%, were still online after midnight. Some reported that their concentration in class had suffered as a result of their late night activity.
The most popular online activities were playing video games (23%), using social networks (18%) and watching videos (17%). Of the 19% who said they played war games online, a significant number said they played age inappropriate titles such as the 18-rated Black Ops and Modern Warfare.
The time children spent online and the activities they indulged in was putting them at risk, said Mr Wilson.
A significant proportion of children who answered the survey, 18%, said they had met in real life friends they had made online. Although a majority took along an adult or older sibling to these meetings, a third said they just went along with friends.
Dr Elizabeth Staksrud, from Oslo University, an expert on children's use of the web who advises the EU Kids Online project, said most face-face meetings with online friends were positive experiences for children.
"It's rare that such meetings are associated with abuse," she said, adding the caveat that parents often did not know that such a meeting had taken place.
However, she added that, in a small number of cases where grooming was taking place, it was often not immediately apparent to a child that their online friends could present a threat.
Mr Wilson echoed this comment and said younger children were not applying the real world rules that kept them safe, such as being wary of strangers, when they went online.
"They are very knowledgeable but they do not have the adult life skills to understand what is risky ," he said.
Nickie Forrest, head teacher at Normandy Primary School in Bexley, said parents should be doing more to help their children understand the risks and stay safe.
"Parents have to start taking responsibility for a world perhaps that they don't understand," said Ms Forrest.
'Collaborative approach' needed
In the second survey, commissioned by the Anti-Bullying Alliance, 60.5% of parents said that cyberbullying had become part of life for children and young people.
But 40% of parents and 44% of the teachers surveyed acknowledged that they did not know how to respond to cyberbullying.
The survey of 2,200 parents, children and teachers across England revealed growing calls for online safety to be taught in more schools, with 69% of teachers and 40% of young people saying it should be included in the national curriculum.
Luke Roberts, national co-ordinator of the Anti-Bullying Alliance, said cyberbullying is one of the biggest issues facing young people today and called for the government, parents and social networking sites to come together to teach young people how to stay safe online.
"We need a collaborative approach to tackling cyberbullying, so children themselves can take responsibility for their own safety online and know where to turn for help when things go wrong," he said.