Thirteen-year-old Dennis leads me through a narrow maze of alleys to a cramped shack built from thin metal sheets, plastic and cardboard, showing me where rats have nibbled holes in the walls and roof. A quiet and thoughtful boy, he has opened up enough to tell me he is a huge Wayne Rooney fan, and on the potholed streets he demonstrated an amazing talent for backflips.
Now he is showing me the home he shares with his parents and three siblings: a lean-to with no sanitation, so the open sewer outside often overflows into their house.
"When it rains very heavily you cannot find a place to sleep because of the water coming in," Dennis says, with a resigned shrug. "You are inside but it is like you are outside. We just take it and live . we try to live."
This is Mukuru, a slum on the outskirts of Nairobi, Kenya. It is typical of the informal settlements where more than half of the capital's 4 million residents live.
I have travelled to Mukuru to meet children and film them and their families, to raise awareness of the National Cooperative Housing Union, a project sponsored by Comic Relief. The scheme is giving slum dwellers across Kenya the chance to construct new toilets, improve their homes and even save money to buy new dwellings away from the slums.
My visit lasts just two days but for these children the slums are a life sentence. Yet the optimism and courage they display are staggering, if heartbreaking.
The most overwhelming problem is sanitation. Hundreds of families share filthy communal toilets, which empty into the river that runs through the slum. Dennis shows me human waste floating in the water. The drinking water for this population of about 500,000 comes from a pipe that runs through this river of effluent. When children fall ill, families' poverty is compounded by the need to buy vital medicine.
Dennis, however, is more terrified of fire. With no electricity, people have to use oil lamps to light their homes and infernos often rage through the narrow alleyways, igniting house after house. Not long ago, Dennis came home to find that his previous house had burned to the ground. The family lost everything. "A big fire is like hell," he tells me.
Today he practises his Rooney-style kicks enthusiastically, but I notice he is running about the rough, uneven alleyways in worn-out flip-flops. His football boots, like his school uniform and school books, were destroyed in the blaze.
"I just started crying because I thought my life was over," Dennis says of the fire. "My books carried my future."
Like many of the children here, Dennis has dreams: he longs to be a doctor, not least because he would like to educate others about the importance of clean drinking water. "When I grow up, I want to change my family's life and the lives of the people that live here," he says. "I would like to live in a good house with electricity, a house where you can sleep well, where the rain and rats don't come in through holes. I wish I can get out of this slum one day."
He represents what Mukuru, and settlements like it, could be about: helping children to live healthy lives and fulfil their dreams. I am amazed at Dennis' awareness of the importance of education - not many kids I know in the UK would say their most prized possessions are their books. Dennis fills me with both optimism and enormous sadness.
Later I meet eight-year-old Robert, a smiley boy, full of curiosity, who just wants to play. He lives in a small shack with his parents and three siblings; his treasured companion is Kitty, a cat.
Robert also lost his first home in Mukuru, through a forced eviction in the middle of the night. He was only 4 at the time but he vividly remembers waking to the sound of tractors bulldozing homes, people screaming, grabbing possessions and running for their lives. "I heard a noise, `do-do-do'. My mum woke me and said there was a tractor coming to demolish our home," Robert says. "I was really scared and angry." Friends came to help but amid the chaos they could save only what they could carry. "The next day, our house had gone."
In a sign of hope over experience, his mother Mary does her best to make their shack homely, hanging lace sheets over the corrugated iron.
Just like Dennis, Robert longs for a proper house with a kitchen and toilet, so that he will no longer have to cross a busy road and pay to use the stinking public latrines. I am struck by how quickly this little boy has had to grow up. It is brilliant that a child of his age wants to go to school but scary that such a young boy understands that without a good education he will be trapped in Mukuru, or somewhere like it, forever.
But there is something inspiring about Mukuru. I find it astonishing that, despite the squalor, the poverty and the brutality of overnight evictions, people have such spirit. They may be forced to live in difficult circumstances but they are not broken by them.
It shocks me to think just how close my family could have come to this situation, although my grandparents were middle class when they emigrated to Britain from Ghana before I was born. But visiting Mukuru makes me realise how much common ground we share. People there have the same needs and ambitions as we do, but their situation makes it so much harder for them to achieve their goals.
I hope that children in Britain will watch our film and see how even a little contribution can make a big difference.
Reggie Yates is an actor and former radio DJ, and hosts The Voice on BBC One.