As you drive north from Nairobi to Embu, the soil is bright red and the landscape lush with banana crops and emerald fields of tea, coffee and rice. Del Monte, the canned fruit people, grow pineapples here, and because it's October the purple jacaranda trees are in bloom.

But once past Embu, the provincial capital of Kenya's Eastern Province (from where you can see Mount Kenya, its snowcaps recently melted by global warming), things begin to change. The tarmac road gives way to arid scrub, which is only passable with a four-wheel-drive. Along the road we pass biblical scenes of herdsmen and women poking their wayward goats and skinny humpbacked cattle with sticks, and camels being walked down from Ethiopia to be slaughtered for meat. Decrepit, moth-eaten donkeys pulling carts are beaten by owners with vicious leather whips. I'm no animal-rights activist but it's hard to watch. When we eventually arrive in Marimanti, Tharaka, it feels as if we've travelled back several centuries. It is dark, and all I can see are huddled shacks and groups of shadowy figures lit by the occasional paraffin lantern.

It is serendipity that has brought me here. For, unlike Dr Johnson, I was tired of London and offered myself as a volunteer to the charity Plan International, an NGO that sponsors children in developing countries. I already sponsor a small boy, Mutuwri, in this region. We've written over the years. I've sent him postcards of red London buses and the Houses of Parliament; he has replied with drawings on pages torn from a school exercise book. It was suggested, as I am a writer, that I work in his and other local schools.

I had been warned that Tharaka would be dusty and hot. There is no infrastructure, which means no roads and no water or electricity. The children here often walk eight miles to school, sometimes without shoes, and female circumcision is routinely practised on girls as young as 12. This is often sanctioned by the women in the community and considered a means of reducing sexual appetite and keeping women faithful. The girls then drop out of school to be married off, facing a life of gynaecological complications and manual work. It's not unusual to see a woman trekking along the road with a baby strapped to her front and firewood on her back. As we drive over the dust roads I notice how formally everyone is dressed, despite the sweltering heat. The men wear pleat-fronted trousers and long-sleeved shirts, and the women skirts, blouses and formal jackets, as if going to work in a building society. Poverty, it seems, makes people want to appear respectable.

The Christian churches are strong here. Apart from the Anglican, Catholic and Methodist ministries, there are numerous shacks that proclaim themselves the "Word of God Tabernacle" or the "Apostolic Pentecostal Church". Religion is big business, and many a self- appointed pastor has gone from goatherd to man with big shiny car from tithes paid by his flock. Sunday mornings in Marimanti are a crackle of amplified singing and preaching.

And the schools? Well, they're built of mud and have earth floors. There is no glass in the windows and nothing on the walls except a patch of black paint that serves as a blackboard. The pupils sit in huddles at the few desks, dressed in patched but neat school tunics and shorts, as if they've dropped out of a 1950s children's annual. They jump up when I enter, wishing me good morning and welcoming me to their class. The little ones crowd round touching my white skin, and then turn away giggling.

Only primary education is free here (though in the current election campaign politicians are making unconvincing noises about extending free schooling to secondary level), so that it is not unusual to find someone of 17 in a class of ten-year-olds. The teaching is formal and old-fashioned, and lessons are learned mostly by rote. This means the smaller children chant along merrily while the older ones find it hard to be inventive and think for themselves. In the earth-floor staffroom, where at break the teachers take their sweet milky tea that's been boiled up in a kettle, a fat white hen sits under the desk.

Many poor families keep children home when there is no one else to tend to the goats or plant the crops on which they depend. I suspect that's the case with Mutuwri, who is ten and the oldest boy in his family, as he often misses school. He lives with his mother, Catherine, his three sisters and a little brother in a smallholding of traditional mud-and-grass huts; his father left the family to marry someone else. The family grinds maize by hand on flat stones and chickens scratch round and about, though the place is neat and well kempt.

The day we visit, the women, dressed in their best, greet us dancing and ululating. Presents are exchanged and photos taken under the shady awning of a torn rice-sack, and a cousin's baby leaves a wet patch on my knee. I especially relish the photo of Mutuwri standing in his red London T-shirt holding his red London bus (below). They are a lovely family; the children beautiful and shy, their mother beaming an infectious smile. Catherine has prepared a lunch of gruel followed by chicken, potatoes and chapattis. Hands are ceremoniously washed in a plastic bowl as she asks me what crops I grow in London. Like me, she is a single parent, so there is an immediate bond. She is a strong, beautiful, loving woman doing her best to provide for her children. We may come from different worlds, in effect from different centuries, but as we eat, dance and laugh, it is what we have in common that seems relevant.