‘Slumdog Millionaire’, ‘City of God’... you could make a box office hit from the lives of kids in Kibera, the biggest slum in sub Saharan Africa. Even before they go to school here, children must run the gauntlet of Kibera’s crazy and even violent street life.

A view of Kibera, Africa's biggest slum.

Scientists warn that stress can raise levels of the hormone cortisol, permanently altering the architecture of young brains. But while stress can be a problem, so can too little stimulation – as scientists discover how important interaction is for childhood development.

The adults of Kibera are working hard to offer kids a safe and stimulating haven in pre-schools.

Nasuru and his cousin Maria go to a pre-school run by the church - there are no Kenyan government ones. Pre-school gives Nasuru’s mum time to work. It feeds him, and keeps him away from urban stress and unpleasant strangers.

Kids can develop peacefully in this preschool - but not all Kibera's parents can afford it.

Pre-school is a safe space for the kids. Somewhere they can develop peacefully –and, in theory, become less violent adults. But many parents can’t afford the ten dollars a month fees.

Both Natasha’s parents have died and her uncle can’t afford to send her to pre-school regularly. So at four Natasha has been in and out of pre-school. Natasha spends most of the day hanging around the neighbourhood.

Prof. Sally Grantham-McGregor, of the Institute of Child Health, London, says: "I think there’s millions of Natashas around the world. The majority of children who are poor in developing countries are like Natasha. The staying at home and working maybe, or doing nothing constructive in terms of their development, is the norm I would say."

Experts disagree how critical the first five years are and whether more funding should be diverted to early childhood development. But many of those who set the agenda for global development now regard early childhood as a key priority.

James Wolfensohn, Former President of the World Bank, and now running his own early childhood development institute, points out: "The child of course comes into the world with many gifts but they need to be developed. The synapses in the brain need to be developed, the feelings need to be developed, the ability to have social contact needs to be developed, the creative elements need to be developed, the language needs to be developed. And you can’t leave it till five or six years old. No, this all happens much earlier - and all the evidence suggest that those children that have this stimulation on all those areas, and combined with that health and proper nutrition to make sure that they develop appropriately, that those kids over lifetime will do better than other children."

Patience is one of the lucky children who can attend preschool.

Mable washes clothes for a living. But she's been able to send her daughter Patience to pre-school - and it's made a difference.

"Patience has really changed with pre-school," says Mable. "Now if I call her when she is far away she will come running. I can also send her to the shops and she goes running. When I am sick in bed I can send her to go buy me some medicine and she will run and fetch it."

But for parents and teachers of children like Nasuru, Brian and Patience, pre-school also brings dilemmas. Should it reflect traditional African social values, or the West’s more individualistic outlook?

Sally Grantham-McGregor has an answer: "Well this is a common criticism that we, the West, are imposing practices on Africa that aren’t appropriate - and to some extent it may be true. But if you speak to the parents and ask them what they want for their child, you will be amazed that they want their children to be doctors, teachers, nurses. They are very ambitious for their children."

Meantime four-year old Natasha is out on the streets, receiving no stimulation at all, and there are some 200 million kids like her across the world... So can Kibera really be a model for Early Childhood Development?

Children in Kibera live in unhealthy conditions

James Wolfensohn has the last word: "If you give the kids better stimulation in those early years, they will do better in their primary, secondary, and tertiary years; they will stay in school. The evidence is overwhelming that if you give them better stimulation in the early years and health and education and nutrition, that if you can get to the kids in those years then your task in primary, secondary and tertiary is much better. And so it is not a waste of money, it’s an investment."

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