Cecilia Kang / The Washington Post

WASHINGTON — Silly Bandz and Pokemon cards need to make room for another craze hitting the monkey bars crowd: Facebook.

Young children are flocking to social-networking Web sites that had been the domain of smartphone-toting high schoolers and young adults.

About 5 million U.S. users on Facebook are younger than 10 and 2.5 million subscribers are 11 or 12 years old, according to a recent survey by Consumer Reports magazine.

And the youngest of Web users aren’t just on Facebook. They are logging onto social networks such as Formspring, tweeting their location to the Web, and making friends out of strangers on Disney and other games sites.

That’s a lot of freedom on the Internet for children who can’t ride in the front seat of a car or leave school with a friend without a signed permission slip from Mom or Dad.

Maybe too much freedom, say child advocates and lawmakers, who are pressuring companies to work harder to keep the youngest users offline and to create federal rules that would limit how companies collect information about children on the Internet.

California is deliberating a bill that would give parents the right to demand that social networking sites such as Facebook delete addresses, e-mail accounts and other personally identifiable information about a youth if a parent asks. Facebook, Google and Twitter have launched an assault against the legislation, saying it squashes free expression.

Reps. Ed Markey, D-Mass., and Joe Barton, R-Tex., have proposed a bill that would make it harder for children to be tracked online and limit marketers from gathering information on a teen’s whereabouts and personal information.

“Companies have had their chance and they haven’t done enough. Kids can be targeted by scammers, cyber-bullies and marketers and the companies aren’t doing enough to protect them,” said Jim Steyer, president of child advocacy group Common Sense Media.

He and lawmakers such as Sen. Richard Blumenthal, D-Conn., argue that the great technological minds of Silicon Valley ought to be able to do a better job of verifying the ages of their customers.

Facebook says that’s not so easy.

It sets a minimum age of 13 and purges the accounts of users found to be in violation. Google, Foursquare, Formspring and other social-networking sites set the same age limit. But they argue that there is little they can do if a user who was born in 2001 puts in a birth date of 1970. Verifying age raises a tangle of privacy issues for companies, who aren’t able to use personal information about young users in the first place, experts say.

Facebook says it’s using technology to patrol for offensive content and working with law enforcement to catch child predators. It regularly purges accounts of underage users, but it declined to say how many sites are deleted each day.

“Recent reports have highlighted just how difficult it is to implement age restrictions on the Internet and that there is no single solution to ensuring younger children don’t circumvent a system or lie about their age,” Facebook spokesman Andrew Noyes said.

Even the most careful parents are finding it hard to keep up with the voracious appetites of their children to try out new social media sites.

Stacy Pena, a San Francisco-area mother, said the social pressures to get online can be immense.

The high-tech public relations executive felt comfortable letting her 11-year-old daughter set up a Facebook account after drawing up a two-page Internet use contract that included promises to be kind, to limit her privacy setting to “friends only” and to give her parents access to her password.

But where things spiraled out of control was on Formspring, a three-year-old Web site that has swept through her daughter’s middle school. The site lets users post anonymous answers to questions posed by others. But Pena found a list of disturbing comments on the profiles of her daughter and her daughter’s friends that went beyond the run-of-the-mill gossip that can foment on other parts of the Web.

“Ur fat”

“Name all the slutty things you’ve done”

“You should just go die”

The anxieties around those questions drew Pena’s daughter repeatedly back to the fast-growing question-and-answer site last fall, even after Stacy instructed her to quit the service.

“I don’t know what was worse for her — seeing those comments or being off Formspring and worrying about what else was being said about her,” Pena said.

With 25 million users — most between ages 13 and 25 — Formspring said it has taken extra steps to prevent online bullying and to create a more congenial environment. In April, it began filtering comments for inappropriate language. The San Francisco-based site lets users make their profiles as public or as private as they want. Users can block other users too. But company officials admit they are limited in their ability to sniff out underage participants.

“We will continually improve our security features and expect to launch even more functions to help lead the industry in securing safe, fun and honest conversations on our site,” said Ro Choy, chief operating officer of Formspring.

About 30 percent of its 10 million comments a day are anonymous, and about 0.5 percent of all comments are flagged as inappropriate by the company’s filter. It sets 13 as its minimum age, but says it can’t do much when a subscriber lies about his or her birth date. And its filter probably wouldn’t catch mean-spirited words that aren’t curse words or sexually explicit in nature, Choy said.