Unicef is warning that smartphones are fuelling a "bedroom culture" with online access for many children becoming more personal, more private and less supervised.
The agency, the United Nations' fund for children, says in its latest State of the World's Children report that one in every three internet users in the world is aged 18 and under.
It says the internet is a "game-changer for some of the world's most marginalised children, helping them fulfil their potential and break intergenerational cycles of poverty".
But it says the net is also creating a "bedroom culture".
"Mobile phones enable children to access the internet in the privacy of their bedrooms or from a friend's house," it says.
"The result is online access that is more personal, more private and less supervised."
The report says children are going online at ever-younger ages.
"In Bulgaria, for example, the age at which children first used the internet was commonly 10 in 2010 but had dropped to 7 by 2016," it says.
"In China, children under 10 made up 2.9 per cent of all internet users in 2016, up from 2.7 per cent in 2015.
"In Brazil, the proportion of 9- and 10-year-olds using the internet increased from 35 per cent in 2012 to 37 per cent in 2013.
"It is not uncommon for children who are not yet even teenagers to own their own phones. A survey in Algeria, Egypt, Iraq and Saudi Arabia in 2013 found that age 10 or 12 was the most common age for receiving a first mobile phone. In 2015, age 10 was found to be the common age for a child to first own a mobile phone in the Philippines, while in Honduras it was age 12."
The report says smartphones "are intensifying traditional childhood risks, such as bullying, and fuelling new forms of child abuse and exploitation, such as 'made-to-order' child sexual abuse material and live streaming of child sexual abuse".
"Predators can more easily make contact with unsuspecting children through anonymous and unprotected social media profiles and game forums," it says.
"New technologies – like cryptocurrencies and the Dark Web – are fuelling live streaming of child sexual abuse and other harmful content, and challenging the ability of law enforcement to keep up.
"Ninety-two per cent of all child sexual abuse URLs identified globally by the Internet Watch Foundation are hosted in just five countries: the Netherlands, the United States, Canada, France and the Russian Federation.
"Efforts to protect children need to focus particularly on vulnerable and disadvantaged children, who may be less likely to understand online risks – including loss of privacy – and more likely to suffer harms."
The report also raises concerned about the "digital divide", with 29 per cent of the world's young people not online - mainly in Africa, Asia and the Pacific.
"Besides issues of affordability and accessibility, there is another barrier facing many of the billions of unconnected people in the digital space – namely, the lack of useful online content in their native language," it says.
"This may discourage potential users from trying to go online or prevent them from directly gauging the potential utility and relevance of the internet.
"But it raises a bigger concern, too: namely, that the absence of content that speaks directly to children's diverse cultural contexts and experiences may widen knowledge gaps.
"The internet of today is, of course, far more multilingual than it was at the beginning of the century. But the fact remains that, in 2016, just 10 languages accounted for the majority of websites, with 56 per cent of them in English," it says.
Only 25 per cent of the world's 3.9 billion internet users actually speak English, followed by 20 per cent who speak Chinese, 8 per cent Spanish, 5 per cent Arabic and 4 per cent for both Portuguese and Bahasa Indonesia/Malaysia.
Unicef recommends providing affordable online access for all children but safeguarding them through measures such as setting privacy settings for children at maximum by default, respecting encryption for child-related data and banning the commercial use of children's personal online data.