Her study of their lives, published in the Journal of Adolescence, reveals a succession of grim backgrounds that preceded their horrific crimes.
Nearly half had been seriously sexually abused as children. Many were from families in which there were high levels of violence. Some had been bullied or branded as school failures. A quarter had watched pornographic videos in the days before killing.
Dr Bailey, one of only three specialists in Britain, runs a national consultancy service from a secure unit at Salford, Greater Manchester, and sees some of the youngest but most brutal killers. She has known 20 of them for some years, and she emphasises how violent their crimes were.
"Eleven victims were stabbed, four beaten and five strangled to death. Compared with adult offenders the degree of violence was excessive. Over half of those stabbed to death received more than 16 wounds and one victim was stabbed 70 times.
"The victims strangled were subjected to repeated strangulations and over a third of the victims of both sexes had been sexually assaulted. Two of the bodies had been partially dismembered and there had been an attempt to blind one victim."
Dr Bailey, married with two children of her own, has spent years monitoring the killers, starting, in most cases, before they were convicted, when she carried out psychiatric assessments.
"I have worked with 40 children who have committed homicide, which is more than anyone else," she says. "It has, I suppose, become a speciality of mine, and I tend to see the cases which are not straightforward."
She visits child killers across the country. Most are held in secure or medium-secure units until they are old enough to go to young offender institutions. Of her study of the 20 she has known longest - all but two of them males - she says: "They come from backgrounds of unstable family lives, absent fathers with a history of alcohol abuse, psychopathic disorders, and violence at home. Mothers had a history of depression and found it increasingly difficult to look after the children as they got older."
She says the finding of high levels of sexual abuse is important. "The fact that they have been abused is a significant factor in what they did and one of the most difficult things to untangle in treatment. In the 20 I have known longest, one-third were sexually abused. In the second group of 20, a higher proportion were abused and in some cases the abuse is pre-puberty. In addition to that abuse, many had fathers who were violent towards them from a very young age.
"I was surprised at first by the high abuse rate. I think there is other abuse we are not picking up, particularly emotional abuse and neglect. In people who have been abused, the aggression and violence they later display is against a victim who is more readily available than the person who has abused or angered them.
"Of course, not everyone who has abused goes on to become an abuser. Some get depressed and some have normal behaviour. What we have to find out is why people behave differently."
Contrary to some public notions, the young killers do regret their crimes. "Almost without exception they are troubled about what they have done and they are also very troubled about what has happened to them," she says.
Dr Bailey, who works for the mental health services of Salford NHS Trust, says there is growing concern about the higher number of younger children now being referred.
"There is a worry that the second group of 20 children I have been looking at are younger, sometimes pre-puberty. There are also more with histories of drug abuse and there is more violence in their crime."
After almost 15 years seeing child killers, Dr Bailey says that adolescence is a crucial time for development when children and families with problems need support.
"In nature, the metamorphosis of the larvae into butterflies occurs in a protected world, but young people develop in very different circumstances, and sometimes they are unsafe. Adolescence is the period of life which should be full of promise and possibility and not abuse."
Dr Bailey, speaking in a week when the courts criticised Home Secretary Michael Howard for raising the tariff for Jon Venables and Robert Thompson, the boys who killed Liverpool toddler James Bulger, strongly criticised the way such children are dealt with in the British legal system.
The psychiatrist, who gave evidence at the Bulger trial, said: "Public interest demands reparation and minimisation of future risk, but often public reaction becomes a voyeuristic end in itself. Politicians, policymakers and the public get caught up in a destructive cycle of blame."
According to Dr Bailey, the British legal system denies accused children full access to psychiatric treatment before their trials. "It would be made much easier if, as in other countries, children accused of serious crimes could be brought to a juvenile or family court after full psychiatric and social investigation."
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