When photographer Marieke van der Velden was asked by the Dutch charity Down to Zero to do an awareness-raising project on Thai children who had been victims of commercial sexual exploitation, she was uncertain how to proceed. For obvious reasons, her subjects’ faces could not be shown.

At home in Amsterdam, she wondered: might the children draw masks? She had a go at drawing her own face (“completely impossible”) and took a snap of her husband, posed behind her mask. This made her smile. She decided the young people could choose to draw themselves or someone else – a person they would like to be. They were enthusiastic, she says: it must have been a surprise – a break from their harrowing stories – to be invited to pick up coloured pencils, to hide behind squares of paper. “It was fun,’’ Marieke says (who thinks her earlier career as a primary school teacher may have sparked the idea). This was a slow, benign, unusual idea. As a photographer, she has done reportage for NGOs and had often felt her work had been “too fast”, especially for children, whenever painful stories were being told.

Got, 15. Photograph: Marieke van der Velden

Down to Zero is an alliance of five organisations working in 11 countries, supported by the ministry of foreign affairs in the Netherlands, and its website describes commercial sex abuse of children as a “complex problem with multiple drivers”. Two million children worldwide are said to have been victims of commercial child abuse. Exploitation thrives along with the growth in western tourism and the explosion of the internet (smartphone usage and poverty are a particularly unholy alliance). The charity’s aim is to enable children to defend their own rights, to make communities safer and to enable governments to improve and implement policies that prevent commercial sexual abuse. The charity also works in the private sector and with the tourist industry.

In Thailand, Van der Velden was photographing in two shelters – one a drop-in day centre. Some of her subjects were still at risk because, she explains, it is not a simple swap to move from street to shelter: “Some of them may feel they have a better time on the streets.”

Throughout the project, she was conscious of her privileged distance. She was glad not to be asking any of the children directly (she communicated via interpreters) about what they had suffered. “I felt we are from another part of the world,” she says. “Who am I to ask what happened? Too much for an afternoon.”

Bon, 12. Photograph: Marieke van der Velden

Most children drew other people: a Korean pop star Van der Velden had never heard of, Manga characters… They could not believe she did not know their heroes: what planet was she from?

She laughs and, as we talk, singles out her favourite portraits. Tas, 14, was “a shy guy” who had been trafficked with his cousin to a British man, via an agent who had found them on the beach. They were given $13 to perform sexual acts; this went on for a fortnight. Tas was 11 at the time and is slowly recovering from the trauma. He spent two hours working on his mask, elaborating it with shadows. He was “so proud of it that he seemed to be growing during the drawing”. He wanted to help Van der Velden with the composition, suggesting the addition of plastic flowers.

Beem, 13, with whom she had a special rapport, was a girl with an edge. Her mother was a hard drug user, and Beem “had to put on sexual performances for Asian and western tourists”. As early as 10, she had been roaring through the streets of Bangkok on a motorcycle. She made van der Velden impossibly sweet milkshakes and urged her to drink them. “She was a real street girl, yet also very kind and funny. If Beem is fighting, she will be fighting hard.”

She talks fondly about little Bon, 12. “He was a small boy and this is my favourite photograph.” Bon was insecure about his mask. He kept saying: “I am not sure if this is at all nice.” As an afterthought, he added teeth to his mask so he could bite when necessary.

Fon, 20.

Phed, 13, who had been assaulted by a foreigner in a temple and suffered from behavioural issues, was not taking any chances, insisting on being Godzilla – the ultimate bodyguard. Fon, 20, asked to be able to draw a house instead of a face. It makes poignant sense: she already had two children and was pregnant with a third. The shelter was helping her raise her children and send them to school. She told Van der Velden she wanted to draw a dream house for her husband – once he is released from prison. Som, 16 – last and by no means quietest – behaved as if she was high on drugs (she had always been homeless and, at one time, a dealer). She drew her portrait at speed and would only stay three minutes while her photograph was taken – and then she was gone.

Van der Velden is, in all her work, driven by a fierce sense of the need for equality. She would love to believe that this project would make men think twice about what they are doing, but realistically considers the photographs she has taken as not “harsh” enough. Yet what matters in the photographs, as in their lives, is that the children are respected.

Van der Velden flinches at the knowledge that, for many of these children, sexual abuse was so normalised right from the beginning that it was only later in their lives that they realised what had happened to them, and only then began to understand that they had all been the victims of “such a big lie”.