Not even the ear-splitting scream from an F-16 fighter jet tearing low through the sky at the adjacent Homestead air reserve base is enough to distract the teenagers from their soccer game.

These are the final precious moments of outdoor recreation time before they must line up in single file and silently make their way, under escort and past uniformed security guards, back inside the giant tents in southern Florida that house their cramped classrooms.

As the homeland security secretary, Kirstjen Nielsen, testified in Washington on Wednesday about hardline immigration policies and a surge of migrants entering the US, these children were stuck in limbo as guests of the US government. The migrants were ages 13 to 17, from countries such as Guatemala, Honduras and El Salvador, and had made dangerous journeys from their homelands, in many cases alone, across hostile territory to reach the southern US border.

On arrival in the US, they are arrested by border protection agents and transferred to what is described as a temporary shelter on 50 acres of remote federal land in Homestead, 30 miles south-west of downtown Miami, to await reunification with relatives or sponsors already in the United States. Or, more rarely, to be sent home again if none can be found.

The presence of the camp, the largest of its kind in the US, is controversial, with activists and some politicians denouncing the 1,700-bed facility and its military-style regime for a “prison-like feel” that epitomises Donald Trump’s hardline approach to immigration policy for minors.

The privately run camp’s managers, meanwhile, say this is a “disheartening” description, and insist on its official designation as a facility of temporary accommodation for unaccompanied migrant children.

“This is not a secure facility, this is a shelter,” claimed the camp’s programme director, who declined to be identified for fear of protesters turning up at her home. “We don’t force anything on the children.”

Police talk with demonstrators outside the Homestead facility. Photograph: Wilfredo Lee/AP

Yet she acknowledged that the children were unable to leave the camp, which is shielded from public view by fences and patrolled by armed guards, except upon their release to sponsors, to consular representatives, or into the custody of Ice, the US customs and immigration enforcement agency, when they turn 18.

“Where is a 13-year-old going to go?” asked the director, a civilian employee of Comprehensive Health Services, the Florida-based, for-profit company contracted to run the facility. “They’re going to come in harm’s way.”

When the Guardian toured the camp last week at the invitation of the US Department of Health and Human Services (HHS), 1,621 children were resident there, roughly 70% male, and all of whom had arrived independently and not been separated from their families at the border. HHS said it had recently received orders to increase the shelter’s capacity to 2,350 beds.

The population varies daily, according to Mark Weber, the deputy assistant secretary of human services at the HHS, who said the day before the Guardian’s visit, 238 children had been admitted and 241 released.

Occupying the residential facilities of adisused Department of Labor job corps campus, the teenagers are housed in dormitories.

Their days are spent in lessons, learning English, reading, writing and maths, arts and crafts and science. Three hot meals and snacks are provided each day, with fish, rice and beans on the lunch menu on the day of the visit. The Guardian was not permitted to talk to the children, but one smiling boy claimed that his maths lesson was facil (easy), and another gave a thumbs-up and declared la comida es buena (the food is good) in the large cafeteria.

There are three soccer fields, volleyball and basketball courts, and organised events and competitions such as tug of war and running races. Communal areas also have giant screens for movies and Xbox games consoles which children can access at weekends and as a reward for good behaviour, the Guardian was told.

Two girls give high-fives as they play soccer at the Homestead facility. But there are plentiful reminders that the site is not a summer camp. Photograph: Wilfredo Lee/AP

But if the atmosphere sounds like a students’ summer camp, there are plentiful reminders that it is not. Mixed with the children’s artwork pinned to the walls are notices of procedures for reporting sexual abuse – a reminder of last month’s bombshell claim that thousands of migrant children had been abused in US custody (the programme director insisted there had been no incidents at Homestead in the 12 months it has been open).

There is a strict no-touching rule, meaning that even a child who hugs a sibling could be written up and face disciplinary action. All the children must wake at 6am, seven days a week (lights out is at 10pm), and they are monitored from a central control room 24 hours a day through smart cards they wear on lanyards and that they must scan every time they change location.

They have no access to cellphones or the internet, and personal phone calls, which shelter managers insist are not monitored, are limited to two 10-minute calls a week.

The average length of each child’s stay, during which case workers attempt to locate and vet sponsors, is 58 days – almost three times longer than the 20-day limit for child migrant detention imposed by the 1997 Flores settlement, which Homestead is not obliged to honor because of its designation as a temporary shelter instead of a permanently sited detention facility.

Protesters gather daily outside the shelter.

“This is Florida’s Tornillo,” said Joshua Rubin, an activist who spent weeks outside a similar government shelter for unaccompanied migrant children in the Texas town before it closed in January, two months after a report highlighted “serious health and safety vulnerabilities” at the controversial camp.

Josh Rubin demonstrates in front of the facility. Photograph: Wilfredo Lee/AP

“This is exactly the same thing: an influx centre on federal land, it skirts local regulations, it’s an unlicensed facility.”

There are darker issues surrounding the government’s detention of migrant children, according to advocacy groups who have filed lawsuits against the Trump administration. The Southern Poverty Law Center said it had uncovered evidence that the US office of refugee resettlement was proactively sharing with Ice information that it gained during the family reunification process, in effect using the children as bait to snare undocumented sponsors.

“They get information from the sponsors [and] they would provide that information to Ice, as well as information about anyone who lives in the sponsor’s household,” said Saira Draper, a senior staff attorney with the SPLC.

“We suspected it all along but what cemented it for us was the leaking of a memo that this policy is being used for the purpose of deterring immigration.

“The fact a sponsor would be deported and a child would be detained longer isn’t an unfortunate side effect, it’s the intent of the policy.”

The impact of such prolonged detention on the children can be extremely harmful, according to Shani King, director of the center on children and families at the University of Florida.

“The government is just not good at raising kids,” he said. “Just the fact they don’t know when they’re going to be reunited with their families is obviously considerably more traumatic for children.

“And even when kids are released, few get the kind of services they need to make up for the initial failures of the system.”