Last week's announcement of a national scheme to "block adult content at the point of subscription" (as the BBC's website had it) was a moment of mass credulity on the part of the nation's media, and an example of how complex technical questions and hot-button save-the-children political pandering are a marriage made in hell when it comes to critical analysis in the press.

Under No 10's proposal, the UK's major ISPs – BT, Sky, TalkTalk and Virgin – will invite new subscribers to opt in or out of an "adult content filter." But for all the splashy reporting on this that dominated the news cycle, no one seemed to be asking exactly what "adult content" is, and how the filters' operators will be able to find and block it.

Adult content covers a lot of ground. While the media of the day kept mentioning pornography in this context, existing "adult" filters often block gambling sites and dating sites (both subjects that are generally considered "adult" but aren't anything like pornography), while others block information about reproductive health and counselling services aimed at GBLT teens (gay, bisexual, lesbian and transgender).

Then there's the problem of sites that have a wide variety of content, such as the venerable LiveJournal, which contains millions of personal and shared diaries. Some of these have material that children – especially small children – shouldn't see, but others don't. Is LiveJournal an adult site? It is, at least according to some filters.

Back in 2003, the US-based Electronic Frontier Foundation took the sites most relevant to each major keyword in the US national educational curriculum and checked to see how many of them were blocked by adult content filters that federal law mandates for libraries and schools that receive federal funding. A whopping 78%-85% of material was miscategorised, and tens of thousand of pages that kids should be looking at as part of their education were blocked by the system.

The web is vast, and adult content is a term that is so broad as to be meaningless. Even if we could all agree on what adult content was, there simply aren't enough bluenoses and pecksniffs to examine and correctly classify even a large fraction of the web, let alone all of it (despite the Radio 4 newsreader's repeated assertion that the new filter would "block all adult content".)

What that means is that parents who opt their families into the scheme are in for a nasty shock: first, when their kids (inevitably) discover the vast quantities of actual, no-fooling pornography that the filter misses; and second, when they themselves discover that their internet is now substantially broken, with equally vast swathes of legitimate material blocked.

Presenting a parent who is trying to keep their children safe with the question: "Would you like to block all adult content on your internet connection?" is terribly misleading, designed to play on parental fears and bypass critical judgement. Better to ask: "Would you like us to block some pornography (but not all of it), and a lot of other stuff, according to secret blacklists composed by anonymous third-party contractors who have been known to proudly classify photos of Michaelangelo's David as 'nudity?'"

It's simplistic to say that governments should abide by the principle "do no harm", but it's perfectly reasonable to demand that policies should at least do some good. When our national information policy is turned over to anonymous, unaccountable censorware vendors, we fail to deliver a safe online environment for our children and we undermine our own free access to information. It's a lose-lose proposition.

As a parent, I worry about what my kid finds on the net. At three and a half, my daughter is already old enough to drive a little tablet and check out cartoons on YouTube. Just the other day, I heard some odd dialogue emerging from across the sofa, and I had a peek at my daughter's screen. To my surprise, I found that she had discovered a little interlinked pocket of aggressive, kid-targeted Barbie adverts, uploaded by the official Mattel account, masterfully wrought pester-power timebombs designed to convert my kid into a nagging doll-acquisition machine. What's more, my kid had heretofore only watched ripped DVDs, YouTube cartoons, and CBeebies and had literally never seen a video advert before.

It was a well-timed reminder to me that kids need close supervision when they use networked devices, even ones that access "kid-safe" content (YouTube screens its users' videos for pornography, and offers a crude parental filter). There are plenty of subject areas that require close guidance and supervision when our kids first see them, and there simply isn't any way a parent can rely on Britain's ISPs to stand in for their personal attention and their work to help kids acquire the only filter that can work: common sense and good judgment.