Parents are still largely unaware of the risks their children take on the net, even though 75% of teenagers use the net at home, says a report.
Three quarters have net access at home
A London School of Economics study suggested 57% had seen net porn but most stumbled on it accidentally via spam or pop-ups.
Only 16% of parents thought their children had seen porn online.
Children are aware of net safety, but parents need help in understanding how to talk through good and bad net experiences and risks, said the report.
Many nine to 19-year-olds worried that if they told their parents about negative experiences, their net use would be severely restricted or their parents would overreact.
Steep learning curve
"I am struck by a sense that a significant minority of parents don't know what they do online," report author Professor Sonia Livingstone, from the London School of Economics and Political Science, told BBC News Online.
"And perhaps it is because they see it as a more positive medium. They don't seem aware of the risks."
This lack of awareness meant parents were unable to properly support and guide their children.
The quick rate of net uptake amongst children in general posed a steep learning curve for them, their parents and teachers alike.
Most nine to 19-year-olds used the net frequently, at least once a week, but only for moderate amounts of time, the report found.
With the rapid growth in broadband, many children said they had more than one computer at home.
A fifth accessed the net in their bedrooms and 79% who had home access said they used the net with no supervision at all.
Worryingly, nearly a third, said they had not had lessons at school on how to use the net, even though most used it for homework.
Significantly, the report found that chatroom use was low: only 21% used them, preferring instant messaging programs (IM) instead.
The closure of MSN's global chatrooms last year had certainly contributed to this, said Professor Livingstone.
"But as soon as IM became widely available, that was the application they wanted anyway," she explained.
"There was a very strong preference for talking in peer networks, where they know people. They did not particularly want to chat to strangers."
Parents are not sufficiently aware that their children are still facing significant dilemmas and contradictions online either.
Many children are aware of what they should and should not do online, but their net experiences were contradictory.
Nearly half of UK children, 46%, had given out personal information online, but only 5% of parents were aware they had.
Many had no choice when applications or websites they wanted to use required such details as part of their registration process.
"You can know the rule that it is not safe, but if you go to a site which asks for that information, you have to give that information.
"It is impossible for them to be responsible and under those circumstances and it is hard for them to work out what is reliable and what is safe," said Professor Livingstone.
In order for them to help, parents need to be able to identify the risks and opportunities while respecting children's right to privacy, the report concluded.
"It is clear that parents don't feel they have sufficient resources, information and understanding about what the net is.
"There is a strong tendency to say, well my child knows more about it than me," Professor Livingstone said.
But, she added, parents needed to be aware that the nature of risks and dangers was a constantly "moving target".
Keeping up-to-date by talking sensibly about what children might do online and unpleasant issues they may have to deal with was key.
That way parents might have a better understanding about what the net can offer children and how to deal with problems when they arise, especially as more go online.
More could be done on the policy and net service providers side too. Anti-spam efforts were helping to divert unwanted content away from children.
But even though great efforts had been made by service providers and other groups to give children safety advice, many were still unsure what they should do if they came across nasty content.
Some kind of portal where children knew they could get everything they needed in one place was perhaps needed, she suggested.
The UK Children Go Online report, funded by the Economic and Social Research Council under the e-Society programme, surveyed 1,511 young people aged nine to 19, and 906 parents.