The website allows users to create and dress up dolls in an online community setting. Stardoll bucks the traditional trend of big entertainment-based sites dominating audiences under the age of 12, such as the BBC's CBBC site which garners over 300,000 unique users under 12 on a daily basis.
Alex Burmaster, internet analyst for Neilsen Online, said: "The internet is very much an entertainment resource for young children, mainly due to how well TV broadcasters such as Nickelodeon, Cartoon Network, the BBC and Disney have adapted and extended their traditional offering to the web."
Disney's Club Penguin came in second in the table, with a 29% audience share under the age of 12, followed by Nick (25%), Lego (25%) and the Cartoon Network (24%) rounding out fifth spot.
Among 12-17 year-olds, mobile phone social networking site Frengo has the highest percentage (26%) of UK web surfers, but again bucks the demographic trend.
Online games sites, including RuneScape, FreeOnlineGames, AddictingGames and MiniClip, dominate the list with the greatest affinity among 12-17 year olds, and landed spots 2, 5, 6 and 8 respectively within the Neilsen ratings.
Burmaster said: "As children hit their teenage years, general entertainment sites tend to make way for game-focused sites, which offer a massive range of easily accessible games and ensure that teenage gaming activity extends far beyond the PS3, Wii and Xbox consoles."
Other sites proving popular with 12-17 year-olds include social networking site Bebo, which landed third spot on the Neilsen Online list with 19% of unique users, and educational resource website BBC Learning (19%).