Devil Baby by Martin RowsonMy five-year-old son Saul's favourite game involves him and a friend bouncing around on our bed in the dark while I, face covered with a luminescent mask roughly based on Munch's The Scream and my dressing gown pulled over my head, roar and grope at the kids like a zombie. For Saul this is the purest joy. And I can't help wondering whether his delight has anything to do with his own behaviour which, on occasion, can be pretty monstrous. There have always been two conflicting notions of the nature of childhood. There's the Hobbes school, holding that human beings are born savage and must be civilised by constraint, correction and social education. This approach seems to be supported by the recent spate of horrific cases – Jamie Bulger, the Columbine massacre, the murder of Damilola Taylor. One of the suspects recently charged over the Heathrow bomb plot cannot be named because he is still, technically, a child. A fear of feral kids in hoodies, real little monsters, has been building in our society for decades.

Then there are those who, like Wordsworth, believe that children are born innocent, touched by angels, "trailing clouds of glory", and are gradually corrupted as they develop into adults. So when children are driven to crime, they would argue, it is because of exposure to noxious images and ideas, whether it's death metal or gangsta rap, video nasties or the vile rantings of a preacher. In this scenario it is the adult world, populated by monsters real and imagined, that implants horrors into the pristine mind of the child.

Such a view has good humanist credentials. The Christian church has traditionally propagated the idea that children are born into sin. So to suggest otherwise used to be considered subversive. In his Confessions St Augustine wrote, "no man is free from sin even a child who has lived only one day on Earth." Any pretence to innocence on the part of a baby was not because of the lack of will to do wrong – for Augustine crying and throwing tantrums were evidence of inherent badness – but through lack of strength. Calvin refined the argument somewhat, eleven centuries later, conceding that when children first arrive on the scene they are like animals, so not at first guilty, but they are born with the 'seeds of sin', which, if not rooted out will bear fruit. The solution was a rigorous 'holy discipline' which nipped those seeds in the bud. Such an idea formed the basis for the 'hands on' religious education that still flourishes today.

It was this view which the 17th century philosopher John Locke challenged in Some Thoughts Concerning Education, where he proposed the idea that children were born a tabula rasa, neither good nor bad, but an empty vessel, to be written on by the hand of Reason. Locke was critical of the harsh Puritan disciplinarianism of the day, and recommended a regime which combined diet and exercise with training in wisdom and virtue. It is not clear what his position on monster games would have been.

An extension of this view, which was also an argument against it, came from Jean-Jacques Rousseau in Emile or On Education. Rousseau agreed with Locke that children are born as a blank slate, but argued that being of nature they were in effect inherently benign. As opposed to the religious view of essential corruption, he held that it is society which corrupts. Before striving to pull children into the adult world of reason – something he agreed was necessary and desirable – Rousseau proposed they first be given a chance to learn the lessons of nature, which would endow them with goodness.

Though less of an ardent rationalist than Locke, Rousseau still distrusted the imagination, home to what he saw as corrupting adult fantasies. So he was opposed to indulging children with stories, or with anything that might interrupt the development of reason. Monsters, he maintained, are a product of adult corruption. "Everything degenerates in the hands of manÂ…. He mutilates his dog, his horse, his slave. He turns everything upside down, he disfigures everything, he loves deformities, monsters. He wants nothing as nature made it."

But there are two basic flaws in Rousseau's conception of monsters as something man-made and perverse. Firstly, not all monsters are unnatural. What were dinosaurs? What about the two-ton estuarine crocodile? Have you ever seen a viper fish close-up? Even the deformities of which Rousseau speaks are, of course, natural developments, however horribly fascinating we may find them.

Secondly, though, and perhaps more crucially, Rousseau offers a very one-sided interpretation of what constitutes a natural mind. He advocates that the inherent purity of children should be protected from negative influences such as misleading fantasies and grotesque fantasy creatures. For a start, anyone who's ever had to spend more than a few hours with the average toddler will be aware that it's not exactly akin to a weekend in the Garden of Eden. But even if you accept the pure innocence of the newborn child, it makes no sense to suggest that it can be preserved by cocooning the child from the real world, or indeed the grotesque world of fantasy.

Far from being an aberrant influence, monsters can be ferociously effective in helping a child to negotiate that world. Saul's room looks like a voodoo shrine. Skulls, skeletons, fossilised bones cover the surfaces. There is a basket of snakes, a box of carnivorous dinosaurs, another for the miscellaneous baddies and beasts he has fallen for – Venom, Baal's Minion, Skeletor, Darth Maul – not to mention the pictures of the monsters he has invented himself – Zogs, Agogs and Slugapuffs. Lined up on pegs an array of masks, Cyclops, 'the goaty one', Dracula and (a particular favourite), the devil.

He has learnt through his fascination with these macabre creatures to make a clear moral distinction between monsters which are animals and those which are not. The animals – dinosaurs, sharks, crocodiles, venomous snakes – should not be blamed if they should, say, eat a little boy, because "they can't help it, they are only animals, it's their nature." Whereas if the devil, Mordred or Apocalyse, should, for example, swallow the planet, they would be entirely culpable, given that they are conscious beings. The degree of culpability, and of scariness, is related to the amount of humanness. Dracula, the devil, Roboticus (a cyborg) are scary because they are half or almost human, as opposed to the 25 metre long prehistoric sea creature, the liepluradon – the world's largest ever carnivore, incidentally – which is awesome but not frightening. Saul is very meticulous about these distinctions and should we get them wrong he is quick to correct us.

Facing monsters gives children the opportunity to explore and think about danger – that posed by animals that could kill you – violence and responsibility. It also encourages the categorisation process so essential to humanity. Anthropologist Mary Douglas has argued that monsters, like the biblical beasts and Beowolf, are 'border-steppas', they are anomalous, refusing to fit neatly into any category, and are therefore of particular fascination to humans, big and small. In her reading of Leviticus Douglas shows how animals which defy easy categorisation are rendered 'unclean' (as in the laws of Kosher) and labelled abominations. It is these boundaries, and the way that monsters – also known as mutants, abominations, beasts, freaks – refuse them, that fascinates and captures the imaginations of children.

And monsters can also be a haven of danger and evil when children are so constantly and infuriatingly expected to shape up to goodness. "Parents and teachers are the enemy," proclaimed Roald Dahl, the master of monstrous villainy, who gloried in the smelly, farty, visceral nature of children. "The adult is the enemy of that child because of the awful process of civilising this thing that when it is born is an animal with no manners, no moral sense at all." Dahl instinctively understood that children can handle the idea of evil; that they need baddies and revolting monsters against which they can judge goodness, that of the other characters and their own.

Children don't just like to look at monsters, though. They like to play with them, be them, make up stories about them, talk about them. One aspect of this is about measurement – the monsters are set in relations with other things and objects and baddies, compared, categorised, understood: How long are they, how high? Higher than daddy? Higher than the house? Which is taller: a liopleurodon or a T Rex, a Zog or the Devil? How sharp are his teeth, are they covered in bacteria like a Komodo dragon? In measuring the monster their power to terrify is tamed, mastered and neutralised.

A second aspect concerns story-telling itself. In the placing of the monster in a narrative the child can experience control and agency, a priority about what will happen which is unavailable in the parent-dominated world, for the first time. They can decide if the Teenage Mutant Ninja Guinea Pigs defeat the Succubus and the Incubus, and if they do it with cunning or brute strength.

And it's not just that children can use and learn from fantasies and monsters. In The Uses of Enchantment Bruno Bettelhiem argued that children need the dark materials of fairy stories because they need to make sense – in a symbolic displaced way – of their own feeling of anger, resentment and powerlessness. They also benefit from learning about violence and brutishness because it "counters the widespread refusal to let children know that the source of much that goes wrong in our life is due to our natures". Monsters, in this sense, are a vital resource for acquiring some control, for a being which is learning who and what it is.

So I'm not going to deprive my little monster of his monsters. If he can't learn to confront and cope with evil, how is he going to learn what it means to be good?