Invisible. That’s the chilling new word for vulnerable children who fall through the safety net of child protection agencies.

Children such as two-year-old Keanu Williams, beaten to death by his mother in Birmingham.

Or Daniel Pelka, aged four, starved and beaten to death by his mother and her partner to the point where his appearance when he died was likened to those who failed to survive concentration camps.

And now four-year-old Hamzah Khan, whose mother Amanda Hutton allowed him to starve to death and left his body in a travel cot for nearly two years. Each child not seen and not heard by the people meant to be ensuring their safety.

In both Keanu and Daniel’s cases social workers took their eye off the child and wrongly focussed on the mother. “No professional tried sufficiently hard enough” to talk to Daniel the serious case review into his case found, partly because his first language was Polish.

And in the case of Keanu another serious case review last week found he was failed by those meant to protect him.

Jane Held, of the Birmingham Safeguarding Children Board, said: “No one walked in his shoes. Staff were distracted by his mother’s needs and by taking what she had told them at face value.” In Hamzah’s case his mother had severe drink problems but still had the wit to keep his death invisible.

“Invisible” makes them sound like ghosts, children who barely got a chance of life and each case never fails to bring tears to my eyes.

But, still, lessons will be learned, I’m sure, so that will be all right then. I do not want to criticise social workers and professionals in the other agencies involved in child protection such as the police and the NHS because, let’s face, it’s the killers who killed these children, not those who have joined a caring profession because they care about the fate of children like Keanu, Daniel and Hamzah.

Social workers across the country are overworked, overstretched and underpaid for the responsibilities they take on.

They have gone into their job to help people, certainly not for the money, but their job is becoming impossible.

And when a social worker is dealing with too many cases they are in danger of becoming bogged down with the minutiae of each one, many of which may be very time-consuming because they are borderline dangerous, but with the result they can miss the big picture of one child who disappears into the system and becomes “invisible”, leaving them in a very dangerous place indeed.

One of the culprits in this system overload can be broken down into simple mathematics: the UK has had the biggest population growth of any country in Europe in the past year and its biggest baby boom since 1972, while the number of social work posts fell, for example, by 6% in the year to 2011, according to an annual investigation carried out by the profession’s journal Community Care.

But what’s also gone missing is common sense. It’s the one lesson that is never, ever learned. The sense to realise that in caring for a child’s welfare the very least you should do is see them, speak to them and listen to them.

You need to see all over their home, particularly where they sleep and eat, and to ask them about seemingly trivial everyday aspects of their lives.

Often one little detail that comes up in these tragic cases is that the child has been left in dirty nappies for days or they sleep in soiled cots.

Alarm bells should be ringing. If parents can’t be bothered to change nappies what else can’t they be bothered to do? Feed them? Give them drinks?

Daniel was starved and beaten for months but somehow no one spotted it, even at school, where he was scavenging in bins for food, because his mother was a “manipulative liar” and everyone believed her.

It is also common sense that a small child left in the care of parents who drink to excess and regularly take drugs is, by definition, at risk because addicts put their needs first.

In the case of Daniel both his mother and her partner were drug and alcohol abusers, police were called to the family on many occasions to deal with incidents involving domestic violence and alcohol abuse and the partner had been in jail three times and had been previously arrested for assault.

A little boy at the mercy of two adults on drink and drugs and one a violent criminal? Just those facts alone should have rung very loud alarm bells.

Perhaps a rule of thumb for social workers should be: would I feel happy to leave my own children in the care of these people?

Would they leave their own children in soiled nappies for days, allow them to live in houses with dog faeces on the carpet, accept explanations for injuries at face value from people on drink and drugs?

No, I wouldn’t, either.

People like me, an ordinary middle-aged woman with 15 years’ experience of motherhood, should be recruited by authorities such as Brighton and Hove City Council as volunteers to provide a woman-on-the-street’s overview of cases such as Daniel, Keanu and Hamzah. All we need is a little training and in return we can help prevent them becoming “invisible”.