Scarlett Thomas: The tiny voice at your ear that says 'Don't buy that, buy this!'

It probably won't come as much of a surprise to parents that three- to five-year-olds wield so much power over the family purse that they now have their very own demographic group: "Tinies".

A poll commissioned by a children's TV company has found that what used to be termed "pester power" has mutated into a rather more potent force. A friend of mine who has recently become a father certainly isn't anticipating that in a few years' time his small child will be allowed to help choose a family car or computer. But another parent I know says he finds it inevitable that his six-year-old child will take part in these decisions. Such and such a brand will be dismissed as "lame" or "sad". Another will be "cool".

Some parents are so keen to be "cool" that they will consciously take this advice. After all, with marketing executives nowadays having a direct line into children's brains, you can be sure that the younger the child, the more accurate their judgement of "cool". And with being "young" and "cool" so important in our culture (you can forget about being in a pop group, presenting most things on TV, being a model or even a "young" novelist once you pass the age of about 26), the only way thirty- and fortysomethings can buy into this culture is to take advice from those at its centre.

Playground culture has always led to consensus over what is in and what is out. When I was a child, "in" or "out" didn't affect much more than choice of penny sweets, comics or pop music. And parents couldn't have been less interested. When I was between three and five I was completely unaware of most advertising. Only allowed to watch Playschool on TV, and unable to decipher sophisticated print-advertising messages, my life was blissfully free of "cool", "to die for" and "unmissable" products.

At the age of six I could only recognise one brand: Clarks shoes. The first piece of advertising that influenced me was for Monster Munch crisps. I remember opening that first packet and thinking of the brightly coloured monsters from the commercial. Now, of course, children's imaginations are full of brands and their imagery. Studies have found that branded characters are even used in imaginary play.

The psychologist Sue Keane has been quoted as saying that today's "Tinies", barely more than toddlers, are becoming "savvy consumers". Oh please. These kids aren't savvy consumers. They just respond to savvy marketing.

The problem, is when adult products such as cars, holidays and computers are marketed to and, as a result, chosen by children. I know of a child who pleads for his family to fly on Virgin because of its child-friendly in-flight entertainment. (They do.)

Many parents still only use their computers for email and word-processing. Today's "tinies" are growing up with Snowman CD-ROMs and Barbie Fashion Designer. They are learning the skills to use (and buy) tomorrow's "killer" software and hardware. Another friend of mine will base decisions about electronic goods on the advice of her children. "MP3?" she says. "What the hell is that? And what's a sound card?" Her kids know, of course.

Adults have learnt not to be impressed by claims that a product is "bigger", "better", "magic" or "miraculous". But children believe these things. And what better demographic group is there than the one that believes everything you tell them?