When I first saw Channel 4's populist parenting show, Supernanny, in which horribly behaved children are "tamed" by severe-looking ex-nanny Jo Frost, I found it too distressing to watch in full. Those poor kids, I thought; those awful parents; that terribly intrusive camera. Then I had a child of my own and suddenly saw the point of it. It's all about Schadenfreude, I now think - that little glimpse of a real-life soap opera worse than your own; the sudden, meanspirited rush of genuine relief: thank God my kids aren't as bad as that!

Frost, aka Supernanny, disagrees, of course. "That's part of the attraction for some, I suppose," she says reluctantly. "But I'd hope that people have taken a lot more from it than just that.

The techniques, the advice, the way I help families get through these problems." She looks up expectantly, but I fear I am still looking blank. "I take my job seriously," she insists. "It is all about the families for me. Utmost integrity is key." I tell her I'm questioning my motives, not hers, and she shrugs and pulls an uncomprehending face.

Of course, in the flesh, Frost looks nothing like her alter ego, the cartoon-ish version of a very English nanny with her scraped-back hair, strict black suit, stilettos and spectacles, who has just come back to our screens. But this is the image that has propelled her from her modest start as an unqualified child-minder to fame and not a little fortune.

It all came about when Frost saw an ad in the paper for a television-friendly nanny. At the time she was working as a kind of childcare consultant, helping out stressed mums with advice. She had already been nannying for 13 years, starting as a babysitter in her teens, but had, over the years, grown bored of basic nannying and was on the lookout for a new challenge. She was called for audition and won the part, since when her life has been turned upside down.

"It all feels very natural, like a progression," she protests, when I ask whether her life is now a whirlwind of glamorous TV interviews and make-up sessions. "Yes, from the outside it looks like a whirlwind, but for several years I'd reached a plateau with nannying. I was known as this trouble-shooter and I was taking calls from other nannies asking for advice and calls from parents across Europe and America. I clearly saw an opportunity with Supernanny to reach not just one family but lots of people, and to give everyone the benefit of my knowledge and experience."

At times, it has to be said, Frost sounds quite enormously pleased with herself.

It all comes back to that image; without the stern but sexy dominatrix, Supernanny could have been just another family reality show. The carefully angled shot of Frost in a dark tailored suit, wagging her finger officiously has come to define the show, and build Frost's own brand. But it was TV production company Ricochet's idea, not hers. "Though actually," she points out, "the glasses were mine and I do happen to wear lipstick and black eyeliner."

But today, showcasing a tan fresh from Malaga, she is wearing a hot red T-shirt, black three-quarter-length trousers and pretty sandals. Her long wavy hair hangs down her back. She is much glossier than I'd expected - perfect lippy, shiny hair - and certainly looks far better than that awful picture of her currently slapped across packets of Rice Krispies, with a bland message advocating the eating of Kellogg's cereals for breakfast.

The source of the gloss and shine, I suspect, is America, where Supernanny is big business. "The American show is definitely more polished," says Frost. "But I wouldn't say more glamorous. I don't have an entourage. It's a bigger production, so instead of five people on the road, like we have over here, there's maybe a 22-strong crew travelling with me."

The American show's success means Frost now spends several months a year shooting an annual series which alternates with the UK-based one. Although she remains an employee of Ricochet, and the show is firmly their property, it is claimed Frost has made up to £5 million from Supernanny merchandising deals, a Supernanny magazine, website (b4ugoga-ga. com) and a parenting consultancy business for desperate mothers. With her trademark Naughty Step, Mary Poppins starch and sarf London accent, Frost is now as famous in the States as X Factor's Simon Cowell.

"My life is crazy," she laughs. "People think I'm joking when I say I actually live out of two suitcases - one for winter, in case I end up in Chicago and it's minus 13, and the other for hot weather in case I wind up in California. I've got beach shorts to snow boots. People think I'm living this amazing life, that I'm always in LA, staying at the Beverly Wilshire, but I'm not. Most of the time I'm travelling five hours from one state to another. But I don't do this for the fame or the money. I didn't go out of my way to be famous."

None the less, it's a remarkable achievement. In just four years she has become not only the recognisable face of a booming parenting industry but a star children expert fêted on Oprah Winfrey and David Letterman.

Part of her project involves the publivery-cation of a series of self-help guides, under the Supernanny brand, beginning with a new baby book, Jo Frost's Confident Baby Care. This is a crowded market and clearly Frost is hoping her image will project the book in shops already heaving with similar manuals. To this end, like a cookbook by Jamie Oliver, its pages of text are interspersed with large photographs - Frost dressed in soft pastels and lacy blouses, holding babies, cooing over babies, feeding them, bathing them, mothering them. In one picture, she looks for all the world like the proud mum of year-old twins. Yet, at 37, Frost does not have children of her own.

This isn't an issue, I think, when it comes to disciplining seven-year-olds or putting toddlers to bed - no one questioned Dr Spock's ability to understand precise maternal emotion, after all. But it does make it harder to take seriously advice on breast feeding, "controlled-crying" or post-birth hormones. Frost says, a little tritely, that she's been a nanny for 17 years and that "you don't need to have a child biologically to love it and understand it".

Does she want a child of her own? "People ask whether being Supernanny has put me off kids, but I have to say that's not true. My work is my priority at the moment. It's a relentless schedule, filming here and then in the States, and I'm happy with the choices I've made right now - I can certainly say as a 37-year-old at this precise moment, talking to you, I don't know if I'll have kids. But I really don't believe in this panic over biological clocks. There are many, many parents who are making conscious decisions to adopt children from all over the world, and if I don't have one biologically, then maybe I'll do that. I don't know. I'm not in that place right now." Part of the problem is that she's "living out of suitcases" and too busy to find a partner. Despite the millions she must have made from the Supernanny franchise, when she comes back to England she still lives in south-west London with her father, a builder. "I'm quite a free spirit," she says. "When I've got more time, it would be nice to look for a property, but at the moment I don't know where I'm going to end up settling."

Her mother died of breast cancer at the age of just 43, when Frost was 24 and already a nanny. "People sometimes ask me whether I do Supernanny to honour my mother's life, but that's not it really. What I do, I'm doing for myself - I think it's easiest," she adds, "to just accept that I'm one of many people who have a strong vocation to help others."

But how much does she really help? One of the major criticisms of Supernanny - and similar shows like How Clean is Your House? or You Are What You Eat - is that the trouble-shooting "expert" merely treats the symptoms of dysfunction and not the root cause. It's all very well making a child eat at the table, or stop throwing toys, but what some of these families actually need, you often feel, is a good divorce lawyer, or a course of antidepressants or prolonged therapy. Frost maintains that the "truthfulness" and rawness of family emotion is one of the show's key selling points. But does she not understand people's discomfort at the quick-fix format? "But we do give them that extra help," she replies.

If the rise and rise of Supernanny reflects our voyeuristic interest in reallife dysfunction, it also suggests a crisis of confidence, born of pure ineffectiveness, in modern parenting. Parents feel besieged: in the past few weeks, we've been warned about poisoning our children's food with chemicals, binge-drinking pre-teens, a crime wave among the under-10s and a likely measles epidemic among unimmunised infants, all the implied result of feeble parenting.

Frost's response, you might say, comes from the heart of Middle England, with its emphasis on discipline, consequence and parental control. Much of what she says is undoubtedly common sense: " We want to control children's behaviour, not their emotions. We've already had generations of people who never spoke about feelings and had no emotional connection with each other. But we don't want to go the other way, either, to a family dynamic that's all about expressiveness, with too many choices and not enough boundaries - that doesn't work either. For me, it's about finding the middle g round between the two."

But I hate the idea of "enforcing discipline" by resorting to a catch-all technique, of yanking a three-year-old off to a Naughty Step - we used to call it being sent to stand in the "corner", didn't we? - and the build-up of resentment between parent and child that I'm convinced it must cause.

I shall still watch Supernanny with deep reservations, probably from behind the sofa.

• Supernanny is on Channel 4 on Wednesdays at 8pm. Jo Frost's Confident Baby Care is published by Orion at £12.99.