A bewildered boy sits wrapped in a foil blanket on a concrete floor under strip lights in a bleak, cavernous warehouse.

He is locked inside a wire mesh cage with other boys, watched over by uniformed guards who carry guns and speak a language he does not understand.

Signs on similar cages opposite read “males 15-17” and “males 7-10”. Further along the corridor of makeshift holding pens, girls are held separately.

Of all the dispiriting moments in Daniel Sawka’s downbeat new movie Icebox, the scenes inside the immigration detention facility are perhaps the saddest. Listless, frightened children, eating bad food, held in a stateless limbo.

Icebox arrives on HBO as a timely fictional treatment of events on America’s southern border, where Donald Trump has tried to turn fear of the outsider into an election-winning issue.

The president ignited his conservative base for last month’s midterms by attempting to manufacture a crisis about a caravan of migrants he claimed were staging an “invasion”. He sent troops to the border and fanned conspiracy theories that migrants were being paid by Democrats or were a front for “unknown Middle Easterners mixed in”.

Then as soon as election day had passed, Trump stopped talking about the issue.

But the reality of his exploitation of immigration continues to have an impact on thousands of children who are being held in detention facilities.

Trump’s administration has built a detention camp in the Texas desert which is now said to be bigger than 203 of the 204 US federal prisons. The camp at Tornillo currently holds more than 2,300 boys and girls in a tent city under tight security with little access to mental health care.

There is much to be shocked about in the Trump era, but the deliberate separation of children from their families stands as the cruelest expression of his uncompromising politics.

However, holding children alone in detention facilities predates Trump. It was 2014, under the presidency of Barack Obama, when director Daniel Sawka first saw an image which inspired the movie that lands now amid intense focus on the border and Trump’s assault on migrants.

“I was struck by this boy on a concrete floor wrapped in a space blanket behind a wire fence cage and there were all these other children in the background and I had never seen anything like it and I didn’t understand this space, this predicament,” he said.

“At first, I thought it was some earthquake or catastrophe ... I read it was taking place in Arizona an hour from where I was sitting and I had to find out more.”

Sawka, who is Swedish, has his own family story of forced migration – his father arrived in Sweden as a political refugee in his late teens from Poland, and several generations of his family had to leave their own countries.

He decided to tell the story of what is happening at the southern US border after seeing that image from a detention facility and then learning more from journalists, activists, human rights groups and undocumented migrants.

Sawka’s film follows a 12-year-old named Oscar, played by Anthony Gonzalez, who voiced the character Miguel in the Pixar animated hit Coco.

Oscar is forced into a gang and has to leave his family behind in Honduras as he flees to the US with only his uncle’s telephone number on a scrap of paper stuffed in his shoe.

The movie starts with Oscar being held down while a gang tattoo is etched on his chest and follows his frightening journey with human traffickers who get him across the border and into the US, where he is soon caught and detained.

Inside the detention facility, Sawka illuminates a dehumanising experience for Oscar and the other young detainees, but the guards are not cruel or abusive.

That depiction draws on the border patrol officers Sawka met during his research. He said they “started telling me about what it was actually like to sign up for border patrol to protect your state and then get placed as a prison guard for kids, and how that made you act towards those kids, and how hard it was to have to keep a distance, because if you got involved it would really start eating away at you”.

Sawka said he had chosen not to portray the fictional guards as stereotypically malign figures because the systemic disregard is the more important issue to raise.

“At the heart of it is the question about what childhood is, and is it something we are willing to protect?

“In the US and Europe, all these places are talking a big game about human rights and the right to be a child and then when it comes down to it, the system doesn’t seem to see them as children,” he said.

In the movie, court proceedings are a terrible mismatch, throwing Oscar before a judge who is not so much malevolent as bureaucratically incapable of displaying empathy.

It is an upsetting scene of a man losing his temper with a scared child, failing in his irritation, to get anywhere close to truth or basic humanity.

“The approach seems to be it’s almost like you are a criminal,” Sawka said. “You have to defend yourself without the right to an attorney, you have to provide your own attorney, which is obviously bizarre.”

Oscar and his uncle, who leads a precarious life as an agricultural worker, are left to an uncertain fate.

What does Sawka want viewers to take away from the film? “I’m not a politician, I don’t have answers for all of these questions, but I do think that it’s something we need to consider – how are we treating children coming in from a desperate situation into America or Europe or wherever?

“I find it so terrifying that we are so divided politically that we can’t even look at a child and say: ‘That’s a child who needs help.’

“They become part of this big political discussion that really seems to be demonising them ... that’s what a big part of this movie is, to show how they are just like any other kid, looking for the same things, safety, friends, comfort – childhood, basically. Putting a face to the statistics, that’s the biggest thing we are trying to do.”

The issue of holding unaccompanied children in detention has been inflamed by Trump’s pitiless policy of separation as a deterrent to people trying to enter the US.

Sawka said: “It has not been solved in the last few decades, but I think how it’s being dealt with currently is obviously atrocious and I think the rhetoric of how we approach this is changing for the worse.

“I hope now we are talking about it, maybe we can find some answers to this discussion, but it’s not looking great at the moment.”

• Icebox premieres on HBO 7 December