The use of social networking sites is common among children
One in five British children has met a stranger they first encountered online, a survey suggests.
And as many as one in four 8-12 year olds ignore age restrictions to use social networking sites, such as Facebook, MySpace and Bebo.
The study of 1,030 parents and 1,000 children by online identity experts also found 72% of parents checked their offspring's web surfing habits.
Social networking sites have age restrictions of 13 or 14.
Identity firm Garlik found that a quarter of the parents surveyed said they secretly log into their child's networking page, to check they aren't befriending strangers.
And some 89% of parents interviewed spoke to their children about the dangers posed by social networking sites, and more than half - 58% - said they were more vigilant online now than a year ago.
Busy parents can't be expected to monitor their children's activities all the time
More than a quarter of eight to 15-year-olds questioned admitted they have strangers as friends on social networking sites and one in five say they have met up with strangers they first encountered online.
And two-thirds of the youngsters interviewed said they have posted personal information on their pages, including details of the school they attend and their mobile telephone number.
Tom Ilube, chief executive officer of Garlik, said "children are at the vanguard of the social networking phenomenon", using such sites such "in the same way other generations used the telephone".
"What you find with young people is that they tend to be a lot looser with their personal information than more canny older people.
"That can be OK if they are in a fairly tightly-controlled environment, but when they are in an environment where they're mixing with people much older than them, then that's something to be quite cautious about."
He said the summer holidays meant children will be spending hours on the internet, adding: "Busy parents can't be expected to monitor their children's activities all the time. What are Facebook, Bebo, MySpace and the others doing to help?"