Many children aged nine to 11 are being very risky online, suggests a survey.
More than one thousand primary school children took part in the research, and it suggests 1 in 5 of them had met someone in person after being in contact online.
Many are sharing personal information and playing games rated for much older children.
They also believe 22% of 9 - 11 year olds had been online after 9pm.
The research has been carried out by the charity Safe and Secure Online.
The survey also asked children about the amount of time they were spending online and what they got up to.
It found that 43% went online everyday and 46% spent more than two hours each time they browsed.
Many (22%) regularly used it after 9pm and a small percentage, 7%, were still online after midnight. Some children said that their concentration in class had suffered because of the late nights.
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.
A number 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, said most face-face meetings with online friends were positive experiences for children, but some were not.
Mr Wilson said younger children were not applying the real world rules that kept them safe, such as being wary of strangers, when they went online.
Meanwhile, a second survey by the Anti-Bullying Alliance suggests 55% of young people accept cyberbullying as part of everyday life.
The survey of 2,200 parents, children and teachers across England showed that more and more people would like 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.
Security experts have urged parents to help keep their children safe by being more involved with what they do online.
"Primary school children know far more about the internet than their parents do," said Tim Wilson, who helped do the research.
He said: "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."