Children are being left to "fend for themselves" against dangers such as bullying and grooming in the digital world, the children's commissioner for England has claimed.
Anne Longfield said that young people who are spending increasing amounts of time online are not getting the level of support required, and called on the government to take action.
A year-long study into children and the internet, titled Growing Up Digital and published by the commissioner, claims that "much more needs to be done to create a supportive digital environment for children and young people".
Children aged 3-4 spend an of average eight hours 18 minutes a week online – doing activities from watching films to playing games – while 12- to 15-year-olds use the internet for at least 20 hours per week, according to the report.
This "explosion" of time spent online by children had left parents "hoping that bad things wouldn't happen" because they felt they lacked the "capabilities or the capacity" to deal with any issues that arose, Ms Longfield said.
'Impenetrable' terms and conditions
The study found that children struggled to understand the "impenetrable" terms and conditions of social media sites.
A group of teenagers were asked to read the terms and conditions of photo-sharing site Instagram, used by more than half of 12- to 15-year-olds, as part of the study – but none were able to fully understand what they meant.
Ms Longfield said: "There was a clear comeback from the kids themselves saying, 'I'm staring at the words but they aren't meaningful to me'."
A law firm condensed and simplified the terms and conditions, leaving many young people in the study surprised by what they had signed up to.
Ms Longfield has called on the government to ensure that a search engine's "right to remove" facility, which applies to EU citizens, remains available to children in the UK after Brexit.
She has also urged the government to appoint a children's digital ombudsman to act as a go-between for children and social media companies.
The report further suggests that social media companies should rewrite their terms and conditions so that they can be easily understood by children.
As part of the study, 900 Mumsnet users were asked about their worries about children using the internet.
It found that almost three-quarters of parents were concerned about their children accessing inappropriate material online, and more than half were afraid of unwanted contact by strangers.
Ms Longfield said: "Kids need to be resilient, they need to have the information ready to be able to deal with their time online and know what that means for them and they need to have the power to deal with it themselves.
"The concern is that the reality of existence for children and teenagers is [an increasing amount of time] spent online – and their time online is not regulated, it doesn't have adult presence there to support them and they enter that world without the support and resilience they need to be able to know what it entails and how they can manage their time online successfully."
The report calls for all children to study digital citizenship to prepare them for life in the digital world.
Russell Hobby, general secretary of the NAHT headteachers' union, said: "Schools can play a vital role in protecting children in the digital world by teaching them about their rights and responsibilities online.
"PSHE would provide curriculum space for the discussion of a variety of important issues – not just online safety and digital understanding, but also some of the concerns that children might turn to the internet to learn about, like relationships and sexuality."
He added: "It is only by tackling these sometimes difficult subjects that we can imbue children with the resilience and understanding that will prepare them for life in a digital world. The government must back teachers by making PSHE a statutory part of the curriculum."
A government spokesman said: "The internet has given children and young people fantastic opportunities, but protecting them from risks they might face online or on their phones is vital.
"The UK is a world leader in internet safety, but there is more to do, and we will carefully consider this report as part of our ongoing work to make the internet a safer place for children."