Children's author Michaela Morgan once described how a group of adult male prisoners sat in spellbound silence as she read Maurice Sendak's picture book Where the Wild Things Are.
It's a powerful image - grown men, many of them with violent reputations, entranced by the story of Max, a naughty little boy sent to his room by his mother for wild behaviour. Max daydreams, conjuring up a land of wild things where he lays down the law. He returns to reality when he smells the supper his mother has left beside his bed, which puts the punishment in the context of her love and care.
The prisoners were moved by Sendak's book, and the tale made them seem suddenly vulnerable to the visiting writer. Few of them had ever been read to as children, or seen such wonderful illustrations, and few had ever had boundaries of behaviour so clearly defined.
This is not to suggest that parental failure to set boundaries - that moment of weakness in halting a toddler tantrum in Tesco's with a bag of Maltesers - will lead the child to a life of crime. But growing numbers of children are showing signs of mental and behavioural disturbance - one in five according to a recent report by the Mental Health Foundation. Lack of effective parenting that lays down clearly defined boundaries and sticks to them is at least partly to blame, the foundation concludes.
Asha Phillips, a psychotherapist, offers help for parents who struggle to spit out the magic word in her book, Saying No, published this month. She attempts to show that saying "no" is as crucial as saying "yes" when parents are trying to build loving family relationships and a child's self-esteem.
The presence or lack of firm foundations becomes clear when a child starts school and has to adapt to being one of a group and fitting in with expectations. Asha Phillips believes parents' ability to say no to unreasonable behaviour is more essential than ever during the child's transition to a world that has suddenly become full of rules and tasks. Children need the boundaries and structures from home, she says, to help distance themselves from more infantile responses to school life "which need to be held in check, so they are free to concentrate and learn".
And teachers, of course, are first to feel the fallout from households where the N-word is never spoken. Lemington First School in Newcastle upon Tyne runs parenting classes because of the difficulties staff face in teaching children whose home life has no routine or boundaries. Lemington borders Scotswood, a former shipbuilding area in the city's west end that has high rates of unemployment and family breakdown.
Sally Craigen, the school's head, who also teaches reception, says children starting school without experience of firm parenting have become an increasing problem. "Such children lack concentration, their listening skills are poor, they can't follow simple instructions such as 'let's line up', they find co-operating, playing and sharing with other children difficult.
"Parents who come to talk through problems admit they are inconsistent. They will say no, but then give in if a child creates. Parents' evaluations of our parenting classes are very revealing. They write things like 'I now know that when I say no I have to mean no' or 'I have learned to be firm without shouting'."
Schools face the same problems as pupils get older - increasingly so over the past five years, says Judith Mullen, president of the Secondary Heads Association and warden of Melbourne Village College, Cambridgeshire. "If only parents would say no. Increasing numbers of children are unable to accept the school's boundaries because they have no boundaries at home. Families seem to spend so little time together. Young people desperately want someone to take an interest in them - and that also involves saying no."
Brian Harrison-Jennings, general secretary of the Association of Educational Psychologists, believes teachers, too, are finding it harder to say no; the result is an increasing number of referrals of persistently difficult children to the psychology services.
Lack of limits at home, he believes, has made growing numbers of children more demanding in school, testing teachers' abilities to control a class to the utmost.
"Teachers feel they are having to do the parents' job," he says. He believes parents and schools should work more with each other to maintain consistent discipline policies. "Children are very aware of their own immaturity," he says. "They find comfort and protection from being told no."
Robert Godber, headteacher of Wath-on-Dearne comprehensive school in South Yorkshire, says many parents are "genuinely caring" and aware that what they do and say "will affect their children's attitudes". But some parents of adolescents "take the line of least resistance".
School life has been affected by a growing tendency for children to have televisions and computers in their bedrooms (with parents avoiding the issue of policing what they watch and do). Teachers are having to face children who are tired and irritable - "on a short fuse and unable to cope with challenges in the classroom" - because they have no set bedtime.
Asha Phillips puts it all down to a lack of parental confidence. When it's the end of a working day and you're collecting a tired four-year-old from the nursery and she turns into an angry, red-faced demon demanding the earth, you've got to be feeling pretty self-possessed to put your foot down. When your adolescent has arranged to go out for the third late night in a row, you have to be feeling strong to say no and wait for the fall-out. You have to be confident enough - and involved enough - to know their anger against you is bound to be short-lived.
The realisation that saying no -and being prepared to be unpopular - is an essential part of raising children to feel secure and independent, can help parents develop this confidence. That is what Phillips hopes her book will offer. Boundaries can also protect children, she says. An adolescent whose parent says no, for example, will have the means to withstand peer pressure to do something he or she would rather not do.
As a working parent herself - she is married to the broadcaster and journalist Trevor Phillips and they have two daughters - she understands the urge for those who see little of their children to fulfil their needs as much as possible in the short time available. But this does not necessarily mean always saying yes. She believes the emphasis on parents being positive at all costs has gone too far.
She quotes the case of Charlie - "a very demanding child who screams if he doesn't get his way". She says: "His parents are at the end of their tether because they feel they have no control over his behaviour."
Charlie "insists" that his mother cooks him pasta every day and refuses to let anyone else put him to bed. "The list of his demands is endless." He has become a despot at home because, according to Phillips, his parents treat his wishes as needs and never say no. He gets no practice in dealing with frustration, is fearful of any criticism and deprived of the taste of feeling furious. "His development is stunted," Phillips concludes. He is an anxious child, fearful that if adults cannot stand up to him he is left unprotected against someone stronger than he is.
The book is full of such case studies - of children such as Hari, a bright 12-year-old who is always in trouble at school, and who "slams doors, shouts and disobeys at home". His parents have crumpled under persistent confrontation and feel helpless. Even in therapy, Hari speaks for them. Phillips points out that Hari is frightened of his feelings of being in charge. He needs to have limits redefined so he can once more "be a child and develop at a healthier pace".
She argues: "It is not fair for adults to give responsibility in this way to a child. Bringing up children is a struggle. They can feel frightened by the knowledge that you are not prepared to go through the unpleasantness on their behalf."
Tim Kahn, the author of another recent book, Bringing Up Boys, also devotes a chapter to saying no. He believes the relationships parents have with their children, their sons in particular, can make the difference between them getting on in life or falling into anti-social behaviour and crime. Parents, he says, have to give space to their children's angry feelings and gestures, but this can only be effective if children have the security of knowing who is in charge.
He writes: "It is the task of parents to set boundaries and the task of children to push against them. Some children, often boys, push the boundaries harder than others and, if anything, they need boundaries more than those who push less hard." Parents, he says, have to learn when to negotiate and be flexible, but also when to say no.
Neither Kahn nor Phillips seeks to lay down hard-and-fast rules. Their books offer explorations of the issue rather than a fail-safe script for parents. The plot has to be flexible - it has to change for different periods of a child's development.
Khan argues that "authoritative parenting" has to be balanced with children's need for "positive strokes".
And Maureen Cruickshank, head of Beauchamp School, a 1,700-pupil 14-18 upper school in Leicester, believes negotiation is all-important and that open confrontation can make adolescents, in particular, dig their heels in. She says: "You are walking a tightrope with this age group. You have to inspire them and emphasise the positive. I don't find 'no' a useful word."
'Saying No: why it's important for you and your child' by Asha Phillips is published by Faber pound;8.99'Bringing Up Boys: a parent's guide' by Tim Kahn is published by Piccadilly pound;6.99l Children's names have been changed.