How grim to be murdered outside Lidl. “Waitrose or M&S Simply Food at the very least,” quipped my dark-humoured friend. But from just beyond the police tape, Dee Willis's demise wasn't funny at all. Just a box of pink carnations beside the sticky brown stain and pathetic, blood-soaked tissue dropped by whoever vainly dabbed at the gash in her chest.

Dee was 28 and - so police believe - had been drinking all day before having the barney with a woman friend which, late at night, was terminally resolved with a knife, right under Lidl's stonking great CCTV camera. So not a teenage victim of gang violence, then. A “domestic". The interest of reporters outside her flat chilled. Next morning, a mile away, was the more thrilling prospect of tortured and murdered French exchange students.

By then Lidl had reopened. The pink flowers had gone, with no cellophaned shrine like those guaranteed to our younger dead. No Bebo tributes, tearful marches or mayoral addresses for Dee. At least, enjoying the novelty of being white and female, she sparked a few paragraphs. Unlike the Tunisian guy stabbed, three streets away and two nights earlier, over an unpaid £1.50 internet café bill.

It may not fit the prevailing narrative of wasted youth, but what does Dee Willis's death tell us? That bust-ups between boozed-up broads no longer end in hair-pulling or a chucked bottle? That on a night out, some women in South London routinely pack a blade? That perhaps because knife crime is escalating, there are those who feel it foolish not to carry one. And that life can be taken so casually on the next street to my friend's Georgian house, with its window boxes of red geraniums, funky Antony Gormley street furniture and overpriced cake shop.

The power of the knife is its banality. Guns are illegal (in most circumstances), serial-numbered, maybe not hard to obtain in London or Manchester, but you have to know the go-to guy. Guns are good for only one thing. But how can you legislate against knives when, for every 528 seized by Operation Blunt2, there are a gazillion more in kitchen drawers? After amnesties, sabres and machetes, switchblades and flick-knives are displayed in triumph. Yet you can kill with a £3 fruit parer from Sainsbury's.

Knives have some ancient, hunter-gatherer power over boys. My sons begged and wheedled until they were given Swiss Army penknives so that they could whittle sticks on holiday like their dad. I winced throughout the latest Indiana Jones movie at the young hero - Indie's son - whose bravery and dash is manifested in his brilliance with a blade. Knives make boys feel capable, resourceful, manly and, when backed into a corner, safe.

That is what torments the London parent, particularly those with teenage (or, in my case, nearly-teen) boys. Is this media spotlight on knife culture fuelling a fashion, just as news reports created the “happy slapping” craze? If so, how can we ever let our sons leave the house?

Down at the Willis murder scene, the David Mitchell lookalike from the Daily Mail said to me: “Times readers don't get stabbed.” He added that kids have always had their stuff nicked by meaner kids; he had never got over someone lifting his Munchman console.

When I recall my own moderately rough school - the cries of “scrap on!” outside the gates, the mean girls who'd thump hard, being shoved once down some concrete steps - I realise that the boundaries between playground argy-bargy, bullying and violence have shifted. Now you are classified a bully if you send a former mate to Coventry; a bundle on the bus is assault. Hovering above the normal rufty-tufty is the paranoid and ever-vigilant modern parent.

Now mayor Boris has succumbed to the prevailing parental mood with his statement this week that he'd tell his own four children to pass by any trouble. No doubt he is shaken by the Ben Kinsella stabbing, a half mile from his Barnsbury villa in North London. His neighbourhood shares my own's unsettling mixture of smuggery and lurking violence.

We simply cannot endure any risk to our children. So we tell them - as the police advise - to surrender everything they have to anyone who asks for it. Take my mobile phone, my cash, my Oyster card, just don't kill me.

A West London friend was horrified when her son repelled a bigger boy who demanded his phone on the bus with an old-fashioned “p*** off”. Fighting back is seen as madness. Kids wandering around in iPod earphones - particularly in private school uniforms - are accused of inspiring crimes of envy. (Although few kids these days appear particularly gadget-deprived.) A message of Gandhi-like passivity is preached, so, surely, the villains realise that hordes of docile potential victims can be tapped like human cash machines.

Not that the white-flag approach helped my friend's son when he caught the wrong bus and ended up on a dark street a mile from home. He coughed up his stuff, but they beat him anyway. The police told his mother that they'd heard Peckham gang members were wearing white bandanas until they earn the right to wear red ones by committing a stabbing. Urban myth or chilling craze? Who can tell? Either way my friend, the coolest mum I know about London's dangers, now plans to drive her children around “until they're at least 35”.

Which is what you see every Saturday night in London's high streets, a traffic jam of parents collecting teenagers from bars. Better a stint as minicab driver than a terrified night's wait while your son negotiates the place where alcohol, anger and knife crime meet. We always drive our 6ft boy babysitter back to his house round the corner, a lift I'd have been embarrassed to accept at 17. At least we have the cars and time to protect our kids, unlike the frontline mothers waiting for their sons to walk back alone through gang-riddled estates.

Last week, walking through the flats behind our house, my ten-year-old asked nervously: “Have you ever been mugged, Mum?” No. “Has Dad?” No. I found myself telling him that street crime is rare, he needn't be worried, most people won't harm him. I don't need to tell him yet that your first teenage mugging in London is almost a rite of passage, which - more than any sex education lecture - feels like the real facts of life.

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