It's lunchtime in the leafy grounds of the Kairos School of Inquiry in Randburg, South Africa.

Dozens of children line up excitedly in front of a table laden with large pots of fresh, hot food.

"Today on the menu we have kitchari - a mix of daal and rice, with tomato chutney, halloumi and salad," the school cook says, as she serves the students.

The school decided to introduce a mainly vegetarian food policy as part of an effort to buck a worrying global trend: for the first time, the number of overweight children has overtaken the number of underweight ones.

The UN says the number of overweight and obese teenagers around the world has nearly tripled in the last two decades.

And the number of overweight children aged five to nine years old has increased from 69 million to 147 million.

And South Africa is one of the worst affected countries.

Parents at Kairos have been asked to pack only whole foods in their children's lunchboxes.

Headteacher Marc Loon believes the policy has provided an opportunity to teach students about the importance of healthy eating.

"If all schools were to emulate our intention of being thoughtful and conscious of what children are putting into their bodies... the health of the children would be served," he said.

The growing popularity of convenience food in the developing world is partly responsible for the growth in obesity rates amongst children, according to Unicef.

Trainee lawyer Mamkhabela Mthembu, 23, told the BBC that when she was younger, fast food was a treat.

"I grew up with my grandmother and not in the best circumstances," she says. "Junk food was something that we looked up to because my granny didn't always have money, so it was a token of celebration."

Because of this, she says she never thought of junk food as unhealthy. Instead, it became aspirational.

But that changed when she moved to Pretoria aged 19 for university, where her student residence was on top of a fast food restaurant.

With a busy schedule, she says she found herself choosing convenience food over cooking - and it's had a lasting impact on her health.

"Now I am overweight, it's something that I'm not proud of," she says. "I had bleeding gums as a child from eating too many sweets which I still have today. I'm starting to have breathing problems."

Her struggles led her to become an advocate for Unicef in her spare time. She wants more students to be aware of the damage fast food can have on their health.

Poor and middle-income countries have seen the greatest surge in overweight and obese children.

But whereas in poorer countries overweight children tend to be from wealthier families who can pay for high-calorie foods, in middle-income economies like South Africa more people can afford to go to fast food restaurants.

It has led to an explosion in the number of chains across the country. The South African fast food market was valued at $2.7bn (£2bn) in 2018 and is expected to reach $4.9bn by 2026.

Unicef says more needs to be done to stop these companies from marketing to children and young people.

"Previously, we would always blame an individual for not exercising enough, or not eating healthily," says Gilbert Tshitaudzi, Unicef South Africa's nutrition manager.

"But we know now that that is not really the case. How do you expect an individual to live a healthier lifestyle if their environment does not enable them to do that?"

He says Unicef has advised the South African government to restrict the marketing of unhealthy food to children.

The constant presence of convenience food is an additional challenge for parents who wish to keep their children healthy.

Memory Padi's eight-year-old daughter, Sophia, was diagnosed with rare autoimmune disease ADEM (Acute Disseminated Encephalomyelitis) when she was 18 months old.

Sophia was given steroid shots to help with her symptoms but her mum believes they led her weight to balloon. She now weighs 107kg (236 lbs).

"Her dietitians can see that this is not about food. They say she eats normally," she says.

Ms Padi has put her daughter on a low carb diet, but still struggles to maintain her weight.

Supporters have set up social media pages to raise money for Sophia. Although Ms Padi says she can't afford fast food, well-wishers will sometimes buy it for her daughter as a treat.

"We rarely go out, but when we do, she sometimes asks for fast food. She's a child, so she wants these things."

Ms Padi is currently unemployed and struggles to provide her daughter with the support she needs, including swimming lessons.

She sometimes takes Sophia for walks around their neighbourhood in Alexandra, a township near Johannesburg's northern suburbs, but the traffic and people staring make her uncomfortable.

The South African government introduced higher taxes on sugary drinks in 2018.

But it hasn't stopped the growing obesity rate in children - 22% of children under five are overweight or obese in South Africa, up from 13% in 2016.

Unicef says countries need to improve access to local, nutritious food for children and teenagers.

"South Africa is known to be food-secure," says Mr Tshitaudzi. "But many households don't have money to access healthier options because of our high unemployment rate."

Back at the Kairos School, it's break time. Children eat from lunchboxes filled with fruit, vegetables and sandwiches made from wholewheat bread.

Until governments are able to implement policies that can reverse the surge in childhood obesity, it's left to institutions and individuals to safeguard the health of future generations.