Asia is home to nearly half of the world's population under the age of 18. So in any global assessment of the impact of urbanisation, children and youth will play a huge role in how both Asia and the world changes.

But beneath the facts and figures is a human story about growing up in this rapidly urbanising world. And in many parts of the planet, these children are seen, but their stories go unheard. To attempt to better understand the risks facing them, I visited four Asian cities: Dhaka, Kathmandu, Manila and Jakarta.

I met some of these cities' poorest and most vulnerable children. The majority came from the streets or from the most destitute squatter settlements and slums. Some lived with their parents or carers; others lived in drop-in centres and shelters with fellow orphans and runaways. Regardless of their status, nearly all were involved in some form of child labour, typically giving all their wages to their parents (if they had parents) to cover the basic costs of daily life.

The daily experiences of these children appeared to strongly shape their perceptions of risk. Across the four cities, they reported exposure to a wide range of environmental hazards (food and water-borne illnesses, epidemic disease, traffic accidents, fire and building collapse) as well as natural hazards (earthquake, cyclone, tsunami, flood or heatwaves).

Dhaka

Street children seen through a rain-covered car window, in Dhaka. Photograph: Donald Brown

With a population of around 15.4 million people and an annual growth rate of 3%, Dhaka is one of the most rapidly growing cities in Asia. It is also one of the most densely populated cities in the world.

Its persistent congestion and gridlock provides pedestrians and passengers with an all-too familiar window into the lives of street children, who commonly work as beggars, peddlers, waste pickers, scavengers and musical performers. Children in Dhaka highlighted "dirty living environments" as the primary cause of diarrhoea, while scabies was identified as a recurrent illness due to the regular flood water.

Twelve-year-old Chandi lived on the street with her eight-year-old brother, after being abandoned when her father married a new wife. They would often go without food, clean clothing and shelter, and were forced to shelter under a small umbrella in a public park when the monsoon struck – leaving them exposed to violence and sexual abuse.

"When I was living on the streets, I could not provide properly for my brother," Chandi says. Another 14-year-old street child tells me: "Girls face high risk from sexual harassment everyday."

Rising above street level are growing numbers of multistorey buildings, that accommodate many of Dhaka's burgeoning garment factories. Most of the children I spoke to there were child labourers from these factories. They spoke of their very real fear about building collapse, with many factories failing to conform to building codes and safety standards.

This was starkly illustrated by the collapse of Rana Plaza, an eight-storey garment factory, which claimed the lives of more than 1,000 people and injured another 2,500. But Bangladesh is not alone in the hazardous nature of its workplaces.

Kathmandu

Children demonstrate earthquake preparedness at a juvenile drop-in centre, in Kathmandu. Photograph: Donald Brown

Although the smallest of our four sample cities, Kathmandu is the largest in Nepal and one of the most rapidly growing in south Asia, with a population of more than 1 million and an annual growth rate of 4%. Its rapid expansion has led to growing land pressures and increasing congestion, particularly in central areas.

One of the consequences of unplanned growth has been the proliferation of precariously constructed buildings, including many brick and carpet factories. Like Dhaka, most of the children I spoke to in Kathmandu were child labourers.

One 12-year-old boy describes his working conditions. "The brick factory I work in is made in a very temporary way – the mortar that binds the bricks is made from mud and the roof is made from tin. I fear that the building will collapse in heavy rain or during an earthquake." Another child describs how heavy winds during a storm nearly blew the roof off his factory.

Even though many children in Kathmandu work 7-10 hours a day, seven days a week, girls are also burdened with the extra responsibility of unpaid domestic labour. Girls in both Kathmandu and Dhaka say how they have less time for reading, studying and school because they have more chores at home.

As the impact of climate change worsens, the girls are having to take on more onerous tasks, such as clearing the home of water, mud and other damage.

Manila

Children from a squatter settlement located in a cemetery, in San Juan City, Manila. Photograph: Donald Brown

The largest city in the Philippines with a population of 11.9 million, Manila's growth continues to concentrate more people in areas exposed to sea-level rise, storm surges, cyclones and tsunamis because of the city's location in a low-lying coastal zone.

The children I met in downtown Manila describe numerous experiences of coping with such extreme weather events. This was particularly the case for those living in squatter settlements and slums, which are commonly found in floodplains and low-lying areas.

The children I spoke to in one squatter settlement in San Juan City say they are most fearful of tsunamis – but that they wouldn't know what to do if one struck. As one 16-year old girl explains: "Even if we knew where to run, none of us would know when to run because our early-warning system was stolen."

Scavengers and wastepickers built a squatter settlement in a cemetery which doubles as a temporary dumping site for municipal waste. Girls and boys from this settlement describe their exposure to various environmental hazards, ranging from the risk of being hit by traffic when dangling from the back of rubbish trucks, to needle sticks and cuts when scavenging through garbage and dumpsters without proper shoes or protective gloves.

The child wastepickers in Manila also live in constant fear of abuse and harassment by adults and police. All the children I spoke to there had witnessed at least one murder, and many of the girls are concerned about being raped – including Maricel, aged 17.

After her parents died, she was forced to scavenge to support her grandmother and herself – something she says she finds increasingly embarrassing. She only earns 20 pesos (27p) a day. Maricel says sexual abuse is a growing concern for her after she had almost been raped, and although she still dreams of returning to school, that is looking less and less likely now.

Jakarta

Children play football in a slum area of Jakarta, Indonesia. Many street children told the report's author they were worried about being abused by adults, including being beaten by police. Photograph: Mast Irham/EPA

Jakarta is the largest city in Indonesia, with a population of 9.7 million, making it the second largest city in south-east Asia after Manila. Sustained growth has exerted significant stress on the city's infrastructure, while disparities between the rich and poor continue to grow – a familiar trend across all four cities.

Jakarta is located in a low-lying coastal zone, exposing its population to many of the disaster and climate-related hazards faced by Manila. But as in the other cities, the children I spoke to in Jakarta were particularly concerned about physical and sexual abuse, including police brutality.

A 17-year-old boy in Jakarta says: "We need to earn money for education and our lives. We don't know why we are harassed, since we are all well-meaning, and we are not criminals. A 16-year-old bunsker says: "I try not to get caught by police while I perform my music, and I try to act politely to people so they don't do anything to me," . A 14-year-old boy living on the streets says: "Adults are not always listening to our voice. Sometimes our voice is just entering their right ear and exiting from their left one.".

The street children I spoke with in Jakarta generally survive on 10,000-20,000 rupiah (51p - £1.02) per day to buy food. Because street children are commonly viewed as criminals, governments adopt punitive responses rather than policies supporting child protection and risk reduction. Yet many children have been pushed on to the streets because of factors such as domestic violence, family homelessness, disasters and rural-urban migration.

Conclusions

Meeting children in all four cities highlighted that the "urban advantage", in terms of better education and life opportunities, does not hold true for those living in urban poverty. In fact, many of the girls and boys suffered a clear "urban penalty" through the absence of accessible infrastructure (eg safe drinking water and adequate sanitation) and affordable services (eg healthcare and education).

But I also learned how even the poorest and most vulnerable children can be remarkably resilient. In Manila, for example, I spoke with members of a community-based organisation called Youth Bind Together. They are demanding new legislation that requires all factories to disclose the chemical wastewater they discharge into surrounding rivers. The children and youth there are taking a lead role through community awareness activities, including the use of music.

And in Nepal, I spoke with members of the Urban Poor Empowerment Society, a grassroots organisation formed by young people with the aim of mobilising the poorest communities to access loans and negotiate with local governments for better infrastructure and services.

Although children are disproportionately at risk on many fronts, it would be a mistake to think of them only as victims. Even in the toughest urban environments, organisations such as this point towards what young people are capable of achieving as agents of change and resilience.

Donald Brown is an independent researcher working in urban development planning and climate resilience. His report on children in Asian cities was commissioned by Plan International and the International Institute for Environment and Development