On the playground, children pilfer lunch money and push one another around. But in the cyber-clubhouses they're filling by the millions, kids rig elections, sell fake products and scam each other out of every virtual-worldly possession.

Sophia Stebbins recently joined one such online community, Webkinz, which lets its young members create avatars, play games and hang out. The 9-year-old from Irvine, Calif., worked in a virtual hamburger shop, earned virtual cash and bought a virtual bed, couch and TV for her virtual house.

Then one day, she logged in to her account to discover that all of her gear and money were gone. She suspects that another kid swiped her password and sold her things.

"I was a little scared," she said. "Sometimes now, I hesitate to go online."

An estimated 12 million children and teenagers will visit virtual worlds in 2008, according to research firm eMarketer Inc. So it's no wonder that such sites have become big business.

In the past two years, Walt Disney Co. acquired Club Penguin in a deal worth as much as $700 million, and media giant Viacom Inc. bought Neopets for $160 million.

The sites get the parental stamp of approval by closely monitoring their users and trying to keep out grown-ups with predatory intentions. They offer kids a place to play online without fear of being approached by pedophiles and other ill-intentioned adults.

But protecting the kids from one another has turned out to be hard work.

To keep these worlds from turning into a virtual "Lord of the Flies," Web sites are monitoring every word kids type, limiting them to pre-approved dialogue and patrolling the Web sites with employees undercover as kids. Some also are giving children the equivalent of a 911 call, so they can holler for help.

"When you're at school, there's mostly good people, but there are a few people who try to bully and scam you and do nasty things," said Hazel Dixon, 16, from Reading, England. "It's the same in Whyville."

When she was 11, she trusted the wrong person in the virtual world with her password (he promised her an "avatar makeover") and had every dime of her in-game currency stolen.

Sites emphasize again and again that kids should never give out their passwords. But many fall victim to the common scam: They're told that their avatars will look better or that their accounts will be stocked with virtual currency. Instead, their accounts are usually wiped out.

Jen Sun, president of Numedeon Inc., the Pasadena, Calif., company that created and runs Whyville, said there is an upside when kids get scammed -- they learn a lesson about being careful on the Web.

"It's a learning experience for the victim not to be so gullible, not to be motivated by greed, because the scammers use greed against you," she said.

Two researchers at the University of California, Los Angeles, who study virtual worlds were startled by the "seemingly innumerable" ways that children cheat each other. They detailed several in a 2007 paper published in the proceedings of the Third International Conference of the Digital Games Research Association.

According to their paper and the Whyville staff, Whyville veterans often haze newcomers by demanding rent, even though apartments are free. Other players have figured out a combination of keyboard commands that allows them to jump into the virtual cars of strangers, which is normally allowed only by invitation. Users have claimed that elections for the Whyville Senate were rigged through stuffing of virtual ballot boxes.

UCLA doctoral student Deborah Fields, who wrote the paper with professor Yasmin Kafai, said players were much more willing to engage in behavior that they wouldn't in the real world.

"I don't think they feel monitored," she said. "It's way less monitoring than they probably have in school from just the presence of a teacher."

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