The share of online ad dollars going to TV-related sites appears to be dwindling, unidentified sources tell Mediaweek, eroding the domination by Nickelodeon, Disney (NYSE: DIS) and Cartoon Network. Competition against the TV-centric media companies has been rising from companies ranging from ad network Betawave (formerly GoFish) to the iPhone. As kids spend their online time outside the larger companies sites like Disney.com, Nick.com and CartoonNetwork.com, between 50 percent and 75 percent of online advertising is being directed at sites that aren’t tied to TV programming.

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It’s not that the major sites are seeing less traffic— in January Disney.com had 3.6 million unique kids 2-11; CartoonNetwork.com attracted 1.7 million; and Nick.com drew 1.4 million. The shift reflects the fact that kids likely spending more time online in general and perhaps less time watching TV. The major companies say they’ve been getting the message for awhile, as Nickelodeon has sought to build up its gaming offerings, much of which has no direct tie to its cable programming, while Disney has been expanding its international reach with a Russian cable TV joint venture and its German division’s recent purchase of Mamily, a community site aimed at mothers. Still, Disney is not cutting the ties between its web ventures as a promotional device for its TV offerings; last week, the company unveiled its boys-oriented DisneyXD.com, which coincided with the relaunch of the Toon Disney cable channel.