On the playground, children pilfer lunch money and push each other 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.

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.

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.