'At four years old, Natasha hangs around the Kibera alleys all day – both her parents have died, and her fishmonger uncle can’t afford pre-school. Nor does he trust her to spend the day safely at home, so she’s locked out, under the watchful eye of friendly neighbours.

The other three Kibera kids we filmed were luckier – Nasuru, Patience and Brian - all go to pre-school here in Sub Saharan Africa’s biggest slum, in Nairobi, Kenya.

Natasha looks lost. She waits for her friends to come home. She’s not got much to do. She’s under stimulated - and probably overstressed. We know now from the science that neither are a good idea.

According to Sally Grantham-McGregor, of London’s University College, there may be 200 million children under five who “don’t reach their full cognitive potential… 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.”

And in slums like Kibera, kids can take an additional hit: urban stress can raise levels of the tricksy stress hormone cortisol, potentially altering behaviour for years to come. “We have a growing science base,” says Harvard’s Professor Jack Shonkoff, “that tells us that, among the many things that children learn early in their life, is a sense of safety and a sense of threat...

Children who live in violent, threatening environments over-interpret threat in circumstances that other children as they’re growing up might see as neutral.”

That’s one reason why many mums in Kibera now scrimp to send their kids to pre-schools organized by churches, NGOs and community groups (though not by the Kenyan government, which finds it hard enough to meet the goal of universal free primary school). We hadn’t expected to find so many pre-schools in Kibera, but there they were, and my own trudge round, one early afternoon, left me feeling pretty optimistic about how committed Kibera parents were.

I’d been warned about toddler warehouses for stressed-out mums trying to hack a living in Nairobi’s globalized metro-economy. Maybe they do exist, but not the ones I or our producer Alex Gabbay saw.

Avoid urban violence! Build more stimulating pre-schools! Make sure parents know the crucial role of interaction! (They do here, mostly). And then? This is where the dilemmas come in. This is where the academics fall out, offering an excuse - if it’s needed - for political inaction, or benign neglect. If you’re a child of the Enlightenment you might believe in obvious nostrums, like kids under five should be looked after by adults, and shouldn’t be exploited for child labour. But while clearly encoding some basic dangers, these expressions can also be loaded. At least according to some African, and increasingly Western, thinkers.

For example, is a child with a shorter fuse - because of raised cortisol levels - always obviously disadvantaged? Not necessarily so, according to Jack Shonkoff. “This is adaptive - if you live in a violent threatening environment, it is good for you to kind of have a short fuse.”

And now, having swallowed this pill, try some more from the Cultural Relativism lobby. What if kids don’t always need to be looked after by parents - couldn’t sibling care have its own advantages? What if kids benefit from doing domestic chores, or even helping in the family business?

Four year old Patience, for example, wants to help her mum cut onions. Says mum: “if I am not near, I worry she might cut herself. So I give her a blunt knife so she can learn how to cut and be contented that she has helped her mother.”

And what if some African kids scorn fey, creative fantasy play?

Cameroonian child development expert Professor Bame Nsamenang, of the University of Yaounde, says: “Unlike in the West, where children are involved in nonsense play, African children actually, when they play, they are doing livelihood tasks. Their play is around things that sustain the lives of families... I refer to that as ‘responsible intelligence’...”

But there are issues for fans of African-style Early Child Development too. Sarah Eisenhut, a Western volunteer art teacher in Kibera, seemed frustrated by some local norms. “In the West, we’re very much taught to think for ourselves, even question authority... And here you do everything as a group, even in the classroom. So it is very hard for the children to do something on their own."

Of course most kids - say Early Child Development skeptics, or realists, like Dr Jack Bruer - will turn out more or less fine even if under stimulated by Western standards early on. But much of the debate to date has been about Western parents and societies, where harassed mums and dads aren’t scraping a living on a couple of dollars a day. In ‘developing’ countries, such reassurance is less easy to come by.

Sally Grantham McGregor – co-author of an influential 2005 Lancet series – insists that globally 200 million is a fair estimate of those “who won’t develop to their full potential in language and cognitive development and in social skills - and they will be overwhelmed when they get to school.”

If that’s true, the last film in our series asks, what can we do about it? We try to find out as we follow three Thai kids facing their first day at school....

By the way Natasha, we’re told, has gone back to her village somewhere in Western Kenya. Thieves broke into her uncle’s freezer, and stole his fish – and without his business capital, he can’t stay in Kibera let alone pay for her pre-school.'