There are now 13.1m children online, with 4m of these under 12 years old and 9m 12- to 17-year-olds.
The UK has more children online than any other European country and has seen the most dramatic growth in the under-18 online population this year, with a 58% jump in numbers.
The research follows Microsoft's decision last week to axe the chatrooms on its MSN service in the UK because of their use by paedophiles.
The overall European growth rate for children was 27%. Younger children or those under 12 are joining the online community faster than older children in most European countries.
Tom Ewing, European market analyst at Nielsen//NetRatings, said: "As the internet matures, the demographic profile of users moves closer to the European population as a whole and what we're seeing now is families using the internet more. It's likely that this is linked with the growth of broadband access -- perhaps when they're not paying for access by the minute, parents are likely to let children use and explore the web more."
In sites with a higher than average audience of the under-18s, several sites have emerged, which draw high proportions of their audiences from this demographic. For instance, the under-18s' contribution to file-sharing giant Kazaa is disproportionate to their level in the online population as a whole. Mobile phone sites and online kids' communities like NeoPets also make attractive targets for advertisers wanting to reach children.
"Currently, the majority of sites with a large under-18 audience are aiming at teenagers. But it's the under-12s that are the fastest growing internet sector, and any sites that they enjoy and that their parents trust are in a position to grow very strongly over the next 12 months," Ewing said.
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