Websites for young kids always have to tread carefully when it comes to advertising. Kids virtual world Webkinz is finding out just how much care is necessary. The company, which sells stuffed animals that tie into a virtual world, is being targeted by parents demanding a return to its ad-free status, NYT reports. Turns out they thought paying an average $15 per stuffed pet equaled no ads online.

Webkinz come with a code that lets kids join the Webkinz virtual world, with avatars based on their stuffed pets. The site’s traffic has jumped 800 percent this year, reaching 7.29 million uniques in October. The site began experimenting with advertising this past summer for its own products and games. Then, in October, banner ads for the Jerry Seinfeld-animated film Bee Movie began appearing, along with merchandise tie-ins. In addition to criticism of the introduction of ads on a Webkinz independent fan site, WebKinzMom, a group called Campaign for a Commercial-Free Childhood set up an online petition designed to pressure Webkinz corporate parent, Toronto-based Ganz, to drop the ads.

Before its PR nightmare, Webkinz was considered a model by established toy makers like Mattel. Now, Webkinz’s experience may offer a warning of what not to do. Lacking an admission charge has likely spared rivals—such as Nickelodeon’s virtual world Nicktropolis, which features a Wal-Mart (NYSE: WMT) banner on its home page—from the negativity directed at Webkinz.