After years of being seen as the baddies, nannies are now getting their own back. Two former nannies have written a novel creating a mother-monster character out of all their worst employers. But not everyone believes it's a black and white issue.

"You should hear mama on the chapter of governesses: Mary and I have had, I should think, a dozen at least in our day; half of them detestable and the rest ridiculous, and all incubi - were they not mama?" "My dearest, don't mention governesses; the words makes me nervous. I have suffered a martyrdom from their incompetency and caprice; I thank heaven I have now done with them."

- Jane Eyre, by Charlotte Brontë

Nicola Kraus and Emma McLaughlin are no longer nannies. "Thank God", I say, having read their novel, The Nanny Diaries, a publishing sensation that has achieved the ultimate literary garland: the film rights bought by Miramax. They've done this by plumbing their personal experience as saintly nannies to spill the beans and portray a "fictional" mother-monster, Mrs X, who wants little to do with her adorable son, lest it soil her Chanel pumps and Prada coats. Mrs X schedules three cultural activities a day for her three-year-old son (The Frick, The Met, the French Culinary Institute), makes her son eat organic everything, carob instead of chocolate and won't let him watch TV - until the nanny leaves, that is. Then it's anything goes - TV, pizza, the works.

In Nicola and Emma's creation, the apocryphal nanny from hell has found her revenge: the mother from hell who has a bookcase full of never-read childcare books, a kitchen full of $40,000 worth of never-used cooking equipment, a designer Christmas tree that shall never see a child's school-made ornament and a social diary that might have a window for the little mite in 2010, the day he goes to boarding school.

Nicola (27) and Emma (26) have based their novel on their own experience as nannies in Manhattan. Between them, the two New Yorkers looked after 30 families in eight years, in between their academic commitments. That's 15 families per nanny for an average of six months a time. The contracts lasted for anything from two months to two years, Nicola and Emma tell me. If you've ever had a nanny, you'll understand this.

Nannies arrive at the interview telling you, (the prospective employer) that they will be devoted to your children until they reach puberty and beyond. But we all know that, secretly, many nannies don't intend to stay longer than six months, or until the boyfriend gets fed up with their work schedule, or until they emigrate to Australia, whichever comes first.

The "Nicola"s and "Emma"s of this world don't blame nannies for having the staying power of a fly, they blame mothers for hiring them in the first place. We nanny-hiring mothers are too self-centred to spend two minutes in the sandbox, much less five minutes cleaning up sick in the toilet or a sleepless night comforting a delirious child with an ear infection. The mommy-monster of The Nanny Diaries, Mrs X, would rather spend her time in an isolation tank at the nearest spa than nurturing her son.

The Nanny Diaries is like a cartoon - Bridget Jones becomes a nanny and learns about life. Instead of satirising the life of the thirtysomething, Nicola and Emma are poking fun at the self-indulged moneyed classes who can't be bothered to rear their own children.

In real life, we all know the relationship between nanny and mother can be strained. It can work extremely well. I've had one or two good nannies (back in the days when I could afford them) and I know nannies who have stayed with their families for many years. Most do not. Nannying tends, unfortunately, to be a stop-gap for women who are undecided about careers and education and need a short-term income. As in Charlotte Brontë's day, nannying attracts some women who have nowhere else to go.

Some parents treat nannies extremely well - I'd argue they have to if they want to keep them. I can still remember breast-feeding my baby who rested on one arm, while I wrote an article on my PC with the other arm. My "nanny" calmly sat on the sofa thumbing through Vogue magazine. She could have been hoovering or loading the dishwasher, but nannies don't "do" housework, don't you know.

Try to hire a nanny and the first thing you'll be asked is do you have a housekeeper/cleaner (then you'll be asked if you have pets, a luxury nanny flat, nanny car and nanny swimming pool/gym membership). Nannies do childcare only. The fact that children make appalling messes seems to be lost on nannies and nanny agencies alike. So if you can afford to pay a minimum of 400 euro a week for a nanny (plus PRSI and paid holidays), you'd better be able to afford a cleaner as well.

Such hardship, of course, is from the mother's point of view. Take it from the nanny's perspective, and you hear a different story.

"Nan", the protagonist of The Nanny Diaries, takes a job looking after a charming four-year-old boy and soon learns that she is also expected to run errands for Mrs X, which include organising luxury party bags (Mont Blanc pen anyone?) and outlandishly expensive food for dinner parties. While Mrs X's friends get Gucci handbags as party favours, Nan gets earmuffs as a Christmas bonus. In her spare time, Nan also does the dirty-work for Mr X's mistress, who plays lady of the house when Mrs X is away in the country.

Nicola and Emma say they have lasting relationships with many of the mothers they worked with, but not with the Mrs Xs. The typical Mrs X, Nicola and Emma tell me, is a former career woman who has married rich, had a child, and now is a professional wife. She decorates, shops, plans travel and parties, but has little interest in her child.

By choosing to make nanny-hiring mothers monsters in their novel, Nicola and Emma say they were trying to make a point about "boundaries" (that all-purpose American psychobabble word). When there are no boundaries between nanny and employer and the nanny is taken advantage of, the relationship is doomed. The child pays the price, abandoned by a disgruntled nanny, with nothing to look forward to but a childhood of piecemeal surrogate child-care workers. They have a point.

But it's a two-way street. The employer is frequently taken advantage of by a nanny who comes replete with CV and qualifications, then turns out to have little practical experience and less professionalism.

The situation portrayed in The Nanny Diaries, where a spoiled mother expects a nanny not just to take her place, but to excel beyond the mother's own parenting abilities, is probably unique to the upper classes of places such as New York. (I have to qualify that with the knowledge that there are Irish parents who hire not one, but two nannies, day nannies and night nannies, so that they can have as little as possible to do with their children.)

Perhaps there were halcyon days when professional nannies, who, because of social circumstances, could not marry and have children of their own, and so chose to become faithful surrogate parents to their charges.

Or maybe that's just another Victorian myth. Whatever the history, a nanny can never take the place of a parent and nobody should expect her to.

The Nanny Diaries, by Nicola Kraus and Emma McLaughlin, published by Penguin, £6.99 sterling