RYAN Hudey is in the ideas business. He has been working with a bicycle manufacturer and a confectionary company. He is in the early stages of developing a magnet driven "hover bike" and is doing, some thinking about spray on tattoos.

Ryan will have to scale back his product development work later this month in two weeks the summer holidays will be over and he will be back at school. He's 11.

The suburban Chicagoan is one of many American children topping up their allowances by advising major corporations on what products will appeal to their peers. "I just give them really cool ideas that kids would like and stuff that I draw up, like inventions and stuff," explains Ryan who regularly attends brainstorming sessions organised by Doyle Research Associates, a Chicago based market research firm.

With American children aged between 12 and 19 spending more than $100 billion (£63 billion) a year, companies are trying harder than ever to find out what kids want - so that they can then sell it to them. In some areas, school buses are bedecked with ads for hamburger chains and fizzy drinks, while a special cable television channel pipes adverts for kid related products straight into 12,000 of the country's 110,000 schools.

Peter Zollo, president of Teenage Research Unlimited, says big companies are soliciting the opinions of their young target customers more assiduously and taking them more seriously. "Some companies with big kid products had never talked to kids. That is changing.

Zollo's company maintains panels of teenagers ready to give their opinions on different products. For jeans, its researchers look for kids who follow fashion and, more importantly, are followed by their peers. We'll go to a bunch of kids and say, `Who's the coolest kid you know?'"

A number of American companies have effectively cut out the middlemen of market research firms and recruited their own "consultants". The computer software giant Microsoft runs a programme called Kid's Council, through which At taps the opinions of a panel of local schoolchildren.

The kids meet weekly at the company's "campus" outside Seattle to suggest ideas for new products and discuss the way they use computers and the Internet. In exchange they get Microsoft goodies and an invitation to an annual party.

"We basically, like, advised Microsoft about what kind of things to do," says Andrew Cooledge (11), one of the company's consultants. Andrew suggested the company make more games that appeal to both boys and girls - "they're basically violent games for boys or cutesy little games for girls" - and also advised Microsoft to emulate, some shareware programmes, software cheaply distributed via the Internet.

For his ideas, and for appearing in a Microsoft promotional film, he got "this really cool backpack", some free computer software and $250. As part of the programme, he signed a contract relinquishing any royalties to products developed from his ideas. "I think it's a little unfair if we give them the idea for this programme that makes them billions of dollars."

Most companies still tap the opinion of young consumers through market research firms which specialise in supplying articulate and creative kids for product testing or brainstorming sessions.

Doyle Research Associates, for instance, provides a service called "kideation" in which, as senior researcher Tom McGee puts it, children "are used as the springboard to get people looking at things from a kid's perspective". The children get between $30 and $75 for attending each two hour session.

At first, Malt Faden (12), found the "imaginarium", where Doyle conducts its sessions, "kind of cheesy". The room was filled with banners bearing different words and multi coloured chairs. He and the other children were encouraged to play games to get them into "a creative mood" and were then given large pads on which they were told to write down anything they thought, however off the wall.

Matt was not overwhelmed by his fellow brainstormers. "The kind of kids they bring in there are creative but they're like the kind of kids you don't really want to be around. We had to talk about what our hobbies - were and one of these kids said his was watching TV while eating hot, buttery popcorn.

But he has continued to participate in the sessions, attracted by the $30-40 pay cheque for each. Recently, he has been specialising in cereals, an area in which he already has considerable expertise. He has come up with ideas for cookies and cream cereal, a chocolate and peanut butter variation and "one that you don't know the flavour of until you bite into it".

In general, he says, companies show a poor understanding of what appeals to children. "They just find out what TV cartoon shows are doing well and slap the name of the show on the box, but kids just want something that tastes good, they really don't care if it says Garfield on it like they did when they were five."

Unlike Matt, Ryan says he would offer his consulting services even if he were not paid for it. He has always had loads of ideas, he says, and it is fun to tell people about them.

"They wanted me to draw a diagram of a bike and it had all these cool things on it and it was really funky and they really liked me." His mother says Ryan first began spouting ideas at the age of four when he would advise her on ways to "consolidate" her housework.

Ryan says his friends envy his lucrative sideline but he tries not to boast about it at school, though he did appear once on a daytime chat show discussing his unusual job. "They really liked me." He is saving his earnings "for college" and says he is going to be a doctor, "and also an inventor and an artist. That's my main idea".

Joshua Koplewicz nurses rather more modest ambitions. "I wanna make a big difference in the world," he explains matter of factly. "I wanna make a change that people will read about in textbooks 100 years from now and say, `That was an amazing change'."

The smooth talking 13 year old may already have done a lot to change the way the world looks - or at least dresses. For the past three years, he has worked as a consultant to Levi Strauss, regularly passing the jeans company intelligence on "what's in and what's out and what kind of things kids want".

The oldest son of a New York psychiatrist and an artist, Joshua always seemed to be a few months ahead of the next trend. A few years ago he started wearing Stuci surf wear to school "because I could see it was getting popular with the older kids". Within a few months some of the "more stylish kids" in his class were wearing Stuci - by the next autumn everyone was wearing it.

By then, of course, Joshua (10) had moved on. "For some reason I just have a taste of what's in or out. I have no particular theory of why, I guess it's just in my blood. I'm a very good anticipator."

Levi Strauss harnessed his keen sense of style three years ago after he was invited to one of the company's focus groups by a friend who worked in market research. Joshua was interviewed by a Levi's executive who asked him 100 questions ranging from "the smallest details about brand labels or where a button or pocket is to what kind of sports are cool to play".

The company was sufficiently impressed with his answers to hire him to carry out a number of research assignments every year.

Typically, he says, the firm would send him a notebook, a disposable camera and tape recorder and he would be told to comb the city photographing and interviewing kids he considered to be cool. He was also told to write imaginary letters to "a kid in Russia about what kinds of style are in and what are out and what kinds of things kids do and what they don't".

After each assignment, a Levi's executive would visit his family's Manhattan apartment to review his notebooks and listen to his interviews. Then they would rifle through his wardrobe, grilling him on why he chose particular garments and quizzing him on when and where he would wear them.

It was hard work but it paid handsomely. For around 20 hours' work, Joshua would typically collect between $100 and $120, around 10 times his weekly allowance. The job came with little security. When someone from the company called with an assignment, Joshua says, they usually warned that if he did not return their call within 24 hours he would be fired.

"They were pretty blunt." When the company asked him to recommend two stylish kids three years younger than himself earlier this year, he could see the writing on the wall. "I haven't been officially fired or downgraded or whatever, but they haven't really given me a call in a while. I think I'm too old."