Step this way.

I love primary one school photos: rows of neat bright-eyed five-year olds each in a brand new uniform. Sweet? Yes but now they conjure up an image of other wee kids: like Carly, one of the estimated 100,000 children working up to 13 hours a day picking cotton in India. She featured in a recent news report, prizing open fat cotton buds and dropping the contents into a basket. She wore a grubby dress and had no idea where her parents were. Her arms were covered in scratches and she earned 24p a day. Child labour is tightly controlled in modern Britain but we've merely relocated the problem.

There's another image too: a teenage mother called Yasmina who featured in the War on Want "Stitched Up" report and who earns £25 a month slaving in a sweltering Bangladeshi garment factory, so that high street stores can sell school uniform at loss leader prices. A couple of years ago Asda was offering an entire uniform, including shoes, for £10. Essentially such deals mean poor families in Britain are being subsidised by unimaginably poorer families in the developing world. The store had the cheek to flaunt its membership of the Ethical Trading Initiative (ETI) to inquiring journalists. (ETI members commit to no more than a wish list that includes "working towards" paying a living wage.)

It is a reminder of our need to do more to uncover the back story of what we buy. That's a good subject to tackle in Fairtrade Fortnight, which began on Monday, partly because there is a good news story to tell. Fairtrade has bucked gloomy British retail sales figures by growing 12% last year. To build on that momentum the Fairtrade Foundation is urging 1.5m people in Britain to "Take a Step" (register at www.fairtrade.org.uk/step) by committing to buying a Fairtrade item, raise funds or join a campaign. The figure mirrors the numbers now working in Fairtrade initiatives around the world. Fairtrade tea, coffee, chocolate and bananas are now so mainstream they are actually hard to avoid. That's exactly the sort of mainstreaming that campaigners have dreamed of for decades.

Clothing is more difficult, because early attempts were rather frumpy and because the supply chain is long and complex, militating against clear labelling. So a thread of unfairness continues to run through the industry. Yet it takes only 6p on the price of a T-shirt to double a Bangladeshi garment worker's wages.

Three years ago it was virtually impossible to buy a Fairtrade school uniform. Now there are several specialist suppliers and M&S have switched half their cotton schoolwear to Fairtrade. It is also available from Tesco online. Though the Fairtrade label applies only to the growers, ethical companies insist on "fair grown and fair sewn".

In Scotland, Fairtrade is a learning theme across the new Curriculum for Excellence because, as one teacher put it, this "enriches and challenges children's understanding of the world. It can empower them to think and act as responsible global citizens". According to the Scottish Fair Trade Forum, 171 schools have Fairtrade School status and some are ordering Fairtrade cotton schoolwear for parents to buy. Just as you can't teach kids about healthy eating and serve up junk food in the canteen, you can't talk to pupils about Fairtrade and ignore the ethical uniform issue.

It's easy to knock Fairtrade from both the right and the left. Right-wingers say Fairtrade is too small to make a difference and disadvantages farmers on the outside. Free trade is the answer. That's partly true. Cotton prices might reverse their half century of decline if the US stopped subsidising its cotton farmers. But free trade is also part of the problem because it produces cut-throat competition. Fairtrade agreements guarantee a floor price that not only protects producers but benefits communities and sets a good example. The left criticise it as a form of philanthropy and one that conspires with dirty capitalism. My answer would be that there is nothing wrong with businesses making decent profits, so long as everyone along the supply chain gets a fair cut. Fairtrade isn't perfect but it's a start.