In 2007 a Unicef report ranked the UK lowest among 21 rich, industrialised countries for the wellbeing of its children. Although we came a respectable 12th on one criterion – health and safety (happily, our kids don't tend to die in accidents) – on each of the five others, from poverty to antisocial behaviour, we were at or near the bottom. And the young people themselves were not afraid to express their own sense of dissatisfaction. More of the children surveyed in this country than anywhere else claimed that they felt "out of place", "lonely" or even in only "fair or poor health".

Jay Griffiths's question is simple: why do our children feel so unhappy? Her main answer lies in the title of her new book, Kith. Now used only in the phrase "kith and kin", the word has come to mean something like "extended family" or "circle of friends". But it originally referred to a person's "home territory", the country or region where they lived. For Griffiths, our children have been cooped up indoors, imprisoned in front of their screens (whether television or computer), and they have lost all contact with their kith – with the woods and the wilds, the mountains and moors, the rivers and streams. That, she argues, is the heart of the problem.

It's a diagnosis that chimes with other discussions of contemporary childhood. A follow-up study to the 2007 Unicef report interviewed a sample of British children to explore their malaise more carefully. It did not find these kids longing miserably for the next bit of hi-tech gadgetry; a large number said that they would be happier if they could simply go outside more. And many of today's over-40s talk nostalgically about the freedom of our own youth: how we walked to school on our own, and spent long summer days wandering down disused railway lines, swimming in ponds, or making dens in tumbledown sheds. This was so much better and healthier, we insist, than the protected life of the younger generation now. (Of course, we rarely stop to reflect that we are the very ones who have imposed that protective regime on our own children: no dens in tumbledown sheds for them – think of the risks.)

There is a good deal of sharp argument in Kith. Griffiths is excellent, for example, on the relationship between the "contemporary hysteria about paedophilia" and child-focused consumerism. "Stranger danger", and the fear of abuse that makes us want to keep our kids at home, clearly plays into the hands of the toy and gadget industry, eager to flog us entertainments to keep our house-bound charges amused. And she has some spirited paragraphs on our tendency to prescribe ADHD drugs as a "cure" for fidgeting, or to insist that kids wear goggles to play conkers. I am never quite sure how widespread such silly regulations really are (closer to urban myths than to standard practice, I suspect). But, widespread or not, Griffiths expresses her indignation with engaging eloquence, just as she is eloquently inspirational (as well as funny) about other aspects of childhood freedoms and constraints, from fairy tales to schools. In fact one of the most memorable chapters in Kith tells of her own prodigiously early encounters with books, and captures the way that the written word can have an impact on a child, even when they don't fully understand it. She read Freud, she says, before she was 11 – and mistaking The Interpretation of Dreams for a collection of short stories, decided "they were rubbish".

Appealing as these arguments are, the trouble with Kith is that Griffiths just doesn't know when to stop. It's a classic case of a good idea pursued much too far, and of complexity and counterargument brushed under the carpet – so far under the carpet that at one particularly silly moment she likens the modern treatment of children to a particularly vicious form of antisemitism. "It's really ugly," she declares.

Part of the problem is that, on the topic of kids, she is an unashamed romantic. Despite a few genuflections in the other direction ("Children can be mavericks of malice", she concedes at one point), for the most part she holds a sentimental, rosy view of their nature – one that would hardly survive any prolonged face-to-face experience of a boisterous gang of under-10s. Try telling the average parent that, as Griffiths puts it, "there is a space around a child where even the air seems sensitive", or "children are the musicians of thought". Maybe that is how our kids would be, if only we gave them the space to be so; but I very much doubt it. As most parents have known for millennia, the issue is that children are a curious amalgam of little devils full of (for want of a better word) "original sin" that needs to be controlled, and blessed innocents whose autonomy and natural goodness need to be cherished. That's what makes childrearing such a challenge – and the one-sided views of Kith so misleading.

Griffiths is also an unashamed primitivist (as you might guess from her earlier book Wild, which featured her brave journeys to some of the remotest peoples on the planet); and she firmly believes that more traditional cultures have got childrearing right, where the west has gone terribly wrong. It is this belief that underlies her enthusiastic pages of praise for the physical closeness between parent and baby that you still find in many parts of the developing world ("For infants … among the forest nomads in Paraguay, about 93% of their daylight time is spent in tactile contact with their mother or father" and so on). And she comes close to blaming increasingly distant, brutal or rigid patterns of western parenting for rising suicide rates in Norway, or even for Nazi genocide ("All the leading figures of the Third Reich were trained in obedience from childhood"). But is the grass really so much greener on the primitive side of the fence? Only if you turn a blind eye to its awful aspects. Even Griffiths recognises that the positive features she finds in Inuit childrearing have to be balanced against their traditions of selective infanticide of baby girls, very young forced marriages, and many other social practices we would never want to imitate. You'd need a lot of "tactile contact" to make up for all that.

Perhaps the most important theme of the book, though, is the idea that children should be allowed their autonomy, to roam, to explore the wild woods of their "kith" and, crucially, to take risks. "Children need accidents," she writes, "little ones, ideally, accidents the right size, through which they learn to avoid bigger accidents later." We all know what she means: kids cannot know how to manage risk unless they have experienced some danger. But she gives no hint about how we might achieve accidents of "the right size". The truth is that, partly because of all those tedious rules and regulations, Britain really has become safer for children (as the Unicef report recognised, and as a quick scan of any 19th-century newspaper, with its litany of child fatalities from drowning, skating accidents or falls from carts, will attest). The dilemma is that many us might favour, in general, a less risk-averse world – but not if it's our kids that end up at the bottom of the pond.

And, anyway, are we so sure that all children want to follow the "wild" model that Griffiths admires? One of her child heroines is Laura Dekker, the 13-year-old Dutch girl whose plans to sail the world singlehandedly were resisted by the Dutch courts, until she eventually got her way. Griffiths seems to imply that all kids would be like Dekker if we hadn't stamped the spirit of adventure out of them. But what, I wondered, of the naturally timid, the stay-at-homes, the unadventurous, the quiet and the cautious? Maybe the real crime we commit against our children is to force them all into the same space – whether that's to pen them in, or push them out.

• Mary Beard's Confronting the Classics is published by Profile.