More than half of children have used an online social network by the age of 10, according to a study.
Facebook tops the list of sites that children sign up to under-age, with 52% of eight to 16-year-olds admitting they had ignored the official age limit, the Social Age report for online safety advisory website Knowthenet found.
Other popular sites include WhatsApp, used by 40% of eight to 16-year-olds, BBM (24%), SnapChat (11%) and Ask.fm (8%).
The study found that although 59% of children are social networking by 10 years old, just 32% of parents feel "very confident" about helping them stay safe online.
The poll found 21% of children had posted negative comments, starting from an average age of 11, and 26% had "hijacked" another person's account and posted without permission.
Some 43% had messaged strangers, starting from an average age of 12.
The study suggests that children are most likely to post an image or video of themselves online or set up a fake profile for the first time at the age of 11, try Twitter and message a stranger at 12 and try services like SnapChat and Ask.fm at the age of 13.
Two thirds (67%) turn to their parents when they experience difficulties online, the poll found.
While 63% of parents check their child's internet activity at least once a week, more than a fifth (21%) are not confident they could install parental controls, and just under a half (46%) admit not being confident or aware of the school internet policy.
Dr Richard Woolfson, child psychologist and Knowthenet spokesman, said: "The internet offers wonderful experiences for growing and inquisitive young minds. Yet, as social media has removed the barriers between a young person's public and private self, children can become vulnerable, and compulsive online sharing can lead to danger.
"As this study shows, children are gaining access to social media sites at a younger age, which could expose them to content, people or situations that are out of their depth and which they're not emotionally prepared for.
"Parents can no longer protect children by simply trying to limit their online experiences. Instead parents need to maintain an open dialogue and encourage children to share both good and bad online experiences, talk openly and straightforwardly about the risks they may encounter online without scaring them and make sure they keep up with the latest social media crazes and work with their children rather than trying to control them."
:: Opinium surveyed 1,006 parents of children aged eight to 16 online and 1,004 children aged eight to 16 between October 17-24.
Communications Minister Ed Vaizey said: "The Government takes the issue of child safety online very seriously and has engaged intensively with industry. As a result we have set out a series of measures to help parents keep children safe online.
"The four main internet service providers have committed to provide family-friendly filters, which have the option to install automatically on, that will protect all devices in the home. These allow parents to prevent their children from accessing potentially harmful content.
"In addition, the new national curriculum will see children aged five to 16 taught about internet safety in a sensible, age-appropriate way, a really important step to help children and young people understand some of the issues.
"However, there is no silver bullet and social media sites and users also need to take responsibility. Social networks have a minimum age for users that should be followed."