SAN FRANCISCO — It’s every parent’s worst nightmare: Three adults pretending to be teenagers contacted kids on the mobile social networking app Skout and sexually assaulted them in three separate incidents, police say.

Skout Inc. shut down its forum for 13- to-17-year-olds last week and assigned a team of security specialists to determine whether it can make the app safe for teens. If not, the San Francisco company plans to close that forum for good.

The alleged assaults on two girls, ages 12 and 15, and on a 13-year-old boy underscore how tough it can be to keep kids safe on a new generation of mobile apps.

For years, sexual predators have prowled social networks for young victims. Safety experts say mobile social networking apps pose an even greater threat because they combine profile information about users with GPS information from the users’ mobile phones.

“It’s time to say, ‘Whoa! Slow down; what are we thinking here?’” said Jim Steyer, chief executive of San Francisco nonprofit Common Sense Media.

Jen Singer, a 45-year-old parenting blogger from Kinnelon, N.J., says she and other parents educate children about “stranger danger” only to have companies churn out apps that encourage kids to chat and meet up with people they don’t know.

“You might as well put your kids out on the highway with a sign that says, ‘Flirt with me.’ Anybody could pick them up,” Singer said. “Really, what did Skout think would happen when they combined teens, flirting and the ability to locate flirty teens in one easy-to-use app?”

Skout was originally created as a flirting app for adults, and users had to be 18 or older to sign up. It morphed into more of a social networking app that finds people nearby with whom a user can swap messages, photos and virtual gifts and make plans to meet up.

But so many of the app’s users were kids — about 15%, by the company’s estimates — that last year Skout opened a separate forum for teens.

The company says it took precautions. It says that a quarter of its staff is charged with patrolling the Skout community, and that it deploys technology it calls “the creepinator” that checks for pornographic photos, profanity and other inappropriate activity. Skout refuses to say how many staffers it has, but the New York Times, which was the first to report the assault allegations, says it has 75. Even with that many eyes on its service, police say Skout failed to detect the adult predators who gained access to underage users.

Christian Wiklund, the 31-year-old CEO of Skout and a father of two, declined repeated requests for comment. In a company blog post, Wiklund said: “We will not compromise the safety of our community, and right now, our concerns are too significant to simply stand by and do nothing.”

Wiklund had nursed his young start-up back from the brink of failure after it lost ground to Foursquare in the crowded location-based apps space. As a dating and flirting app and then as a social networking app, Skout took off. Some 15 million people in 100 countries downloaded the app. In April, Skout said it was signing up 1 million new users a month, and it raised $22 million in a funding round led by Andreessen Horowitz, a leading venture capital firm.

Its investors say they are standing by Skout, but the damaging publicity could hurt the company.

“This should be a wake-up call to all the companies to step back and look at what they’re doing,” said Hemanshu Nigam, CEO and founder of online safety consulting firm SSP Blue. Nigam dealt with the problems of predators while chief security officer forNews Corp., which at the time owned Myspace.

Some young users are more concerned about losing access to the popular social network. Many used the app much the way they use Facebook, posting photographs and updates on their profiles and adding friends to their “hot lists.”

Florida teen Dawson Adams said his friends are howling in protest over the shutdown of the forum for 13- to 17-year-olds. “It’s been a real big bummer to everybody. Nobody can get on their account,” Adams said.

Adams, who turned 18 last month, spends about two hours a day on the site and is dating someone he met through the app. Still, he said he often encountered adults in the teen forum, and he thinks Skout should pull the plug on the forum if it can’t keep out predators.

“Teenagers need to be with our own age group,” Adams said. “A lot of teenagers today don’t think about what they do before they do it. They just do it. They don’t think about the consequences.”

There has been an explosion of mobile apps that help people meet, flirt, chat and date strangers near them. But although young people have warmed up to the idea, these apps should set off alarm bells for parents because they often involve relaying a user’s profile picture, location and friends, said online privacy advocate Jeffrey Chester, executive director of the Center for Digital Democracy.

“The small screen, ironically, magnifies the privacy problem,” Chester said. “And that’s compounded when it comes to kids.”

Steyer of Common Sense Media says parents should become more digitally literate so they can guide their children.

“You have to teach your kid to be a safe, smart digital citizen,” he said.

Cate O’Malley, 41, mother of two and a writer and photographer from Rockaway, N.J., said “it’s harder than ever to monitor what our kids are doing.”

But she also remembers that the world wasn’t risk free during her own teenage years.

“The same situations could have happened then. We can’t keep our kids in a bubble,” O’Malley said. “But as access in all forms becomes more and more widespread, we need to make sure now more than ever that we continue to have an open and honest dialogue with our kids to keep them shielded from these potential situations.”

jessica.guynn@latimes.com

michelle.maltais@latimes.com

Guynn reported from San Francisco and Maltais from Los Angeles.