A report looking at 40 websites popular with children has found that users were exposed to more than 200 ads, with many commercial messages promoting products unsuitable for children, such as gambling and dating.
The study, called Fair Game? , assessed commercial activity on children's favourite websites, including Cartoon Network, CBeebies, Neopets, YouTube, Bebo, MySpace, eBay and Lime Wire.
Across 40 sites, the report found that when looking at just the homepage and one additional web page a total of 211 online ads were displayed.
Because many of the websites popular with children are actually targeted at older groups - including many social networking and gaming websites - some of the ads were found to be unsuitable for young web surfers.
An examination of 70 of the ads found on children's websites found that 25% promoted products or services that are prohibited to children under 16 in the UK - including gambling and dating.
The report argued that some of the online marketing used hidden persuasion techniques in the form of ads and commercial messages that cannot be easily identified by children. Almost a quarter of ads were integrated into the content pages,
as opposed to being on a separate part of a webpage that is clearly an advertising slot, such as a banner, the study found.
However, the report also found that commercial content on websites is not a top priority for parents, who focus on other areas of online security, such as bullying, virus attacks and grooming by paedophiles.
The study urged the Advertising Standards Authority to be more pro-active in making sure online users know they can make complaints to the watchdog about internet ads.
The report also called for an education campaign, targeted at schools and parents, to make them aware of the commercial nature of the online environment.
Brands were urged to consider providing ad-free versions of their websites in return for modest subscription fees.
However, the Advertising Standards Authority argued that the report is flawed. The ASA regulates paid-for online ads but not websites created by companies specifically to promote particular products and brand - a "loophole" the report
said should be closed.