SASABE, Ariz. — The 15 migrant children, weary and hungry, stumbled toward a gap in the rust-colored border wall that soars between Mexico and Arizona, nearing the end of their two-week trek north. Unexpectedly, a man in a cap emblazoned with a blackened American flag — traditionally, a message that “no quarter” will be given to the enemy — approached them and coaxed them to his campsite.

Soon, the girls and boys, who were from Guatemala, were sitting under a blue tent devouring hamburgers and sausages. Their host for the day in this remote part of the Arizona desert, Jason Frank, an enthusiastic follower of the QAnon movement, distributed “Let’s Go Brandon” T-shirts featuring an image of President Biden. Giggling and confused, the children changed into the shirts and posed for a group photo. Later, they formed a prayer circle with Mr. Frank and the rest of his team before the Border Patrol showed up.

Mr. Frank and his group, guns holstered on their hips, have been camping out near Sasabe, Ariz., as a self-appointed border force with the stated aim of protecting the thousands of migrant children who have been arriving from the evils of sex trafficking — a favorite QAnon theme.

They are the latest in what over the years has developed into a cottage industry of dozens of armed civilians who have packed camouflage gear, tents and binoculars and deployed along the southern border.

Mr. Frank, a QAnon influencer whose Facebook page in recent months has shown him pictured with such conservative celebrities as Donald J. Trump Jr., Michael Flynn and Sidney Powell, has fashioned his team into a new style of border enforcers, motivated not so much by halting immigration as by guarding the country from other perceived threats — in this case, an unfounded conspiracy theory that migrant children are being funneled into pedophilia rings.

“They are being trafficked, sex trafficked. That’s the No. 1 trade,” Mr. Frank, 44, said as he name-dropped from his list of purported conspirators, starting with the late Jeffrey Epstein. “The money, that’s where it’s at now,” he said.

The federal government has long had concerns that the hundreds of thousands of migrant children who have made their way alone across the border over nearly a decade could be vulnerable to criminal exploitation, and it has put into place an intensive vetting effort to ensure that the young immigrants share legitimate connections with the relatives or family friends who come forward to take them.

But minors crossing the southern border as part of sex-trafficking schemes is unusual, according to groups that monitor and combat trafficking.

“We haven’t heard about migrant children brought in to be sex workers or slaves,” said Stacey Sutherland, an official with the Arizona Anti-Trafficking Network. “At the border, it’s overwhelmingly people who paid to be smuggled.”

Federal officials declined to comment on the QAnon group’s activities, and it was unclear whether the volunteers had broken any laws.

For leaders of QAnon, suspicions that migrant children are falling into the hands of sexual predators fit neatly into the movement’s core conspiracy theory — that an elite cabal of pedophiles led by prominent Democrats is preying on innocent children, an elaborate fantasy that gave rise to the PizzaGate drama during the 2016 presidential campaign. But the new focus on immigration, analysts say, also serves to drum up political support and raise money by tapping into people’s inherent instinct to protect children while promoting hard-line border policies.

“The kids are a prop for them to use to spread their message,” said Mia Bloom, an expert on extremist radicalization and the co-author of “Pastels and Pedophiles: Inside the Mind of QAnon.”

“They are instrumentalizing the children for internal propaganda and to further their political agenda,” she said.

Mr. Frank, who is from Las Vegas, had already become a minor celebrity in conservative circles after helping to carry a 100-year-old World War II veteran to the stage during a Trump rally in Arizona in 2020. His photos and videos have since reached thousands of supporters across a number of social media platforms.

He arrived in Sasabe in late April towing a borrowed recreational vehicle, which he has been sharing with his adolescent son, other QAnon followers who have cycled through and two large dogs. Inside, he keeps a cache of weapons including pistols and a loaded AR-15 rifle, according to his social media posts.

One day recently, Mr. Frank volunteered information and answered questions about his mission before deciding that he did not want to be interviewed by The New York Times. His personal website states that, after drug addiction and prison life, he found purpose in saving children.

Mr. Frank is inserting himself into one of the most complicated aspects of American immigration. While U.S. authorities have been turning away large numbers of migrants under a pandemic-related public health rule, children who arrive unaccompanied — usually carrying an address and phone number of a relative in the United States they hope to join — have typically been allowed to enter the country. Families from Central America, hoping to free their children from the poverty and gang violence at home, often pay smugglers to route the children through openings in the border wall, knowing that Border Patrol agents will pick them up.

They are then put in shelters run by the Department of Health and Human Services, which conducts background checks on the adults who come forward to take in, or “sponsor,” the children. The agency says it cares for the children “until they are appropriately and safely released to a vetted sponsor.”

Most families probably did not anticipate that Mr. Frank and his crew would set up their own ad hoc screening process.

Parked at a location where gaps in the border wall make it easy for smugglers to send in groups of as many as 30 children at a time, Mr. Frank and his team typically greet the young people with hamburgers and hot dogs and broadcast their arrival on Facebook Live, announcing an intention to keep them safe.

Humanitarian volunteers and immigration activists working in the area said they had been dismayed to see that the children, obviously clueless about Mr. Frank and his beliefs, were being diverted before the Border Patrol picks them up.

“We believe the conduct of this group is illegal and extremely dangerous,” said Margo Cowan, a public defender in Pima County, which includes Sasabe, and a longtime immigration activist. She said the law required those who find children alone to immediately contact a law enforcement officer. (Mr. Frank said his group always contacted the Border Patrol after ministering to the children.)

She said she was particularly alarmed at Mr. Frank’s claims that his group was asking children to provide the addresses and phone numbers of the family members or family friends they planned to join, then contacting those individuals, supposedly to keep the children from falling into the wrong hands. These actions could be seen as harassment of adult immigrants who are receiving the children, she said.

“We have people that call and do welfare checks and keep showing up to make it uncomfortable for them,” Mr. Frank said, referring to the adults who ultimately take the children home with them.

Mr. Frank criticized the government’s screening program, calling it “very wide open with a lot of loopholes.” He added, “That’s why we are out here creating a solution, being a part of it.”

In photos posted on another team member’s Facebook page, Mr. Frank and his colleagues at the camp could be seen cradling an infant, who he said was 30 days old and had recently crossed the border with his young mother.

Members of his team called the man whom the mother said she was planning to join, Mr. Frank told The Times. He said that the group had discovered in its research that two of the four people living at the man’s address had ties to organized crime cartels — claims for which he did not offer proof.

Chris Nanos, the sheriff of Pima County, called the “QAnon types” at the border “nut jobs” but said they were not his responsibility.

“If they are interfering with migrants crossing, Border Patrol should deal with it,” he said, noting that he had a million people across 9,200 square miles to protect.

Migrants are not the only ones who have become targets of the QAnon group’s monitoring activities. On April 25, humanitarian workers were visiting the border wall with a film crew from Tennessee, among them a man who is a U.S. legal permanent resident from Guatemala. Mr. Frank and his team spotted them.

“They drove up to us, screaming, ‘Illegal alien! Illegal alien!’” recalled Gail Kocourek of Tucson Samaritans, who runs a resource center that offers food, clothing and first aid for migrants in the tiny town on the Mexican side of the border.

A chase ensued, with Mr. Frank and another QAnon member trying to force her off the road, according to Ms. Kocourek, who said that they stopped when a Border Patrol vehicle crossed their paths. The agent asked the Guatemalan man for his documents.

One of the team members later uploaded a video of the incident to Facebook, which showed a vehicle following closely behind Ms. Kocourek’s car along a desert road. “Who has time to dig,” Mr. Frank wrote, into “little old ladies running ops for the cartel out here? I have names, addresses, ages, phone numbers already.”

The 15 migrant children who had been led into the QAnon camp last week, some of them appearing no older than perhaps 12, sipped water and munched on granola bars as Mr. Frank got the barbecue going.

A Cuban man who had crossed with them was handed a piece of paper and told, through a Spanish-speaking supporter on the phone, to go child by child, taking down their names, their destinations and the names and numbers of the people receiving them.

The children told a reporter that it had taken them 15 days to complete the journey from Guatemala to the United States over land. They had not eaten since the day before, and they were very tired. They appeared bemused, some of them giggling nervously as Mr. Frank mispronounced words in Spanish.

One of the men working the camp was Justin Andersch, a QAnon vlogger who made headlines this year when he accosted Gov. Steve Sisolak of Nevada in a restaurant, threatening to “string you up by a lamp post.”

Mr. Andersch smiled at the gathered children. “Who wants cookies?” he said.

Following the food, T-shirt distribution, photo op and prayer, Mr. Frank handed out Spanish Bibles and telephone numbers for the children to call, should they need anything. “Gracias,” several replied. One boy kissed the holy book.

Several minutes later, Border Patrol agents showed up, loaded the children into a van and sped off.

A couple of days later, Mr. Frank announced on Facebook Live that he had to leave the wall to take care of some business, and promised to return in two weeks.

“We are building our little army,” he said. “So get ready.”

Sheelagh McNeill contributed research.