There was a procedure laid down.

When Alan Johnson was orphaned aged 12 he was to be placed in a foster home. His 16-year-old sister Linda would go to Barnardo's. The two of them were to be separated, as if the death of their 42-year-old mother had not been distressing enough.

If that had happened, would Alan have risen from poverty to become a MP and then Home Secretary? As we now know, he'd have been more likely to end up in a homeless shelter, or in prison. Foster children who are separated from their siblings tend not to thrive. The evidence is unequivocal about the damage that can be done.

Fortunately for Mr Johnson, his feisty sister argued until she was given her own council flat. She cooked, cleaned, paid rent and took care of her brother - until he was old enough to get his first job unloading deliveries at Tesco.

Their story played out in 1963. Today they would be less likely to get their own flat. They would probably join the 302 children in Scotland and 3,582 across the UK who were parted from their brothers and sisters last year. The figures were published yesterday by the charity Action for Children. As a design for destroying the future prospects of our most vulnerable children, it takes some beating. So why aren't we outraged that this cruelty continues and that fostering continues to be a Cinderella service?

Why is it teachers, charity staff, paediatricians and social workers are salaried - and foster parents are not? Why is fostering still regarded as a "vocation" in a world that turns on money? Bankers' bonuses are necessary because we need to attract the best, we are told. Don't we need the best foster carers too?

Instead they receive a payment to cover the child's needs with a small allowance for themselves. However, foster parents tell, uncomplainingly, how they treat the children as their own and can be out of pocket. They rescue vulnerable children when the system doesn't stack the odds against them.

Splitting up siblings is stacking the odds. We shake our heads in horror at the historic shipment of children to Australia. We cringe about abuse suffered in children's homes across the land. We presume our generation knows better and does better.

And then we take "into care" traumatised children whose parents can't look after them - and at the same time separate them from their brothers and sisters, sometimes for life. How is that even legal?

Action for Children explains the situation by saying there are too few foster carers. They are having a recruitment drive because they fear many suitable people rule themselves out unnecessarily. They think they won't qualify because they are single or gay or older or live in rented accommodation. Not so.

I spoke yesterday to an experienced foster carer in Scotland who started taking in children after her own family flew the nest. For the past few months she has been looking after two brothers aged seven and 10. She says they are still frightened. She has given them a shared bedroom so they can look to each other for support, talk in private, share their anxieties - even argue and scrap like normal kids. She knows what they really want is to go back to their parents because - neglectful though their home was - "it is their version of normal".

After nine years of fostering, this substitute mother has come to understand the complexities of looking after traumatised children. She says sometimes it would be too much to take on every sibling. It can be a tough job and the right psychological help for the children might not always be available when it's needed - though she finds the fosterers' network very supportive.

Would domestic assistance make it more possible to take on a family, I asked? What about someone to help with the chores? She sounded as if I was speaking a foreign language or suggesting a fairy godmother. But yes, she said, of course it would make it easier. Clearly such a notion had never entered her head, or anyone else's. It entered mine because I was listening to Woman's Hour yesterday and heard Jane Garvey say the Duchess of Cambridge would have her hands full looking after two children under three. Fat chance. She will have help.

So why not help for foster parents? We have an army of carers who daily help the elderly and infirm. Surely we could extend a similar service to whole families in foster care? An extra pair of hands to help iron, clean cook and babysit could mean the difference between feeling able to look after one child or take on a family of three.

What about larger families? Carol Iddon of Action for Children says youngsters often have to be separated because few foster carers have homes large enough to accommodate them. I can understand that reasoning if we were speaking about a litter of pups. But we're talking about depriving already disadvantaged children of the remaining key relationships in their already disrupted lives.

I asked if children were ever kept in their existing home. Is it ever unsatisfactory parents who are told to move out? If the children stayed in their familiar environment, couldn't live-in house parents be hired? That would require legislation, Ms Iddon said. She has seen a case where a disabled child remained at home while the mother moved elsewhere but that was unusual.

She described what typically happens: "When the child is taken from home and simultaneously from his brothers and sisters he (or she) is angry and frightened. He may blame his foster parents for not taking the whole family. He feels abandoned. His anger can often cause the placement to fail. The boy is then shifted to another new home and to a different school - perhaps removing him from classroom contact with siblings. He is disruptive in the new school, may start to self-harm or refuse to eat. He will be moved again. He may be bullied for being different. As a result he will grow more disruptive, mix with the wrong crowd and start abusing drugs and alcohol. Then comes crime, prison…"

It's a well-worn route of this society's creation - at a huge cost financially and in human misery. But right now - knowing the devastating effect it has - we avert our gaze or shrug our shoulders. The numbers are not large: 100 families in Scotland last year. Preventative help should be affordable for this rich nation of ours. But those 100 families are comprised of 302 children who could suffer the long-term damage I have spelled out. That's expensive.

It wouldn't be easy to make the necessary changes but in that favourite phrase of politicians it would be "the right thing to do".

I suggest we make it illegal to split families - except under extreme circumstances, such as serious sibling abuse. Where necessary the state should be obliged to provide a council property large enough to house the family as a unit. And it should offer willing foster carers a decent salary and domestic help to ease the burden.

With that package Action for Children wouldn't need to launch a recruitment campaign and such vulnerable children would have a decent chance of fulfilling their real potential - Alan Johnson style.