The hardest form of grief

It's almost two-and-a-half years since April Jones went missing while playing outside her family home in Machynlleth, mid-Wales. Despite the biggest police search in British history, the five-year-old's body was never found. Mark Bridger, who had a collection of child abuse images on his computer, was found guilty of her murder.

When April went missing, her father Paul's reaction was to go on walks to escape a house filled with police, family and friends. "I found I was close to April, sitting on top of the hill looking down on the situation," he tells BBC Wales's Week In Week Out, "and I found I talked to her a little bit and it just eased the pressure for me."

Paul's wife Coral felt differently. "I just couldn't go out. I couldn't meet people," she says. "I didn't want to know anybody so I stayed most of my life [at home]. I still do some days now."

After April went missing, a police family liaison officer advised Paul to keep a diary of his thoughts. This forms part of a book, called April, by the Joneses, which is being published this month.

Some other parents of murdered children have done the same. Kevin Wells, father of Holly Wells, who with Jessica Chapman was killed by school caretaker Ian Huntley in 2002, wrote an account of what it was like to live through the disappearance, investigation and trial.

In the 1980s, Ann West, mother of Lesley Ann Downey, one of the victims of Moors murderers Ian Brady and Myra Hindley, wrote a book recalling her ordeal in the 1960s. This included identifying the body and having to listen to recordings of her daughter pleading with Brady and Hindley to stop.

"Writing things down doesn't work for everyone, but it can be therapeutic," says Sandi Mann, senior psychology lecturer at the University of Central Lancashire. "You use the words as a record, helping you to chart your progress.

"Some people will want to keep what's written down to themselves, while others will want to talk in public or write books about their experience because they find it therapeutic. They will also want to keep the memory of their child alive."

Child killings are rare. There were 67 victims of murder or manslaughter under the age of 16 in the UK in 2012-13. Of these, eight were killed by "a stranger", according to the Office for National Statistics. In Scotland, there were two victims under the age of 18, while none were recorded in Northern Ireland.

Partly because of the rarity of these crimes, parents can find themselves under the media spotlight. The cases can stay in the public mind for years. The names of James Bulger, Sarah Payne and Milly Dowler are arguably as well known as when they were first reported between 13 and 22 years ago.

Studies show that bereaved parents grieve differently, some indefinitely. Among the emotions experienced can be guilt at having been unable to protect their child.

Just after the death, the fact that a criminal investigation is going on can make the situation more complex, with police liaison officers, whose primary objective is to help the inquiry, and others looking for evidence.

"There is no fixed formula and no predictable timescale" for grieving, says Katie Koehler from Child Bereavement UK. "How an individual responds will depend on their developmental age and understanding, their prior experience of death and how those around them are responding in addition to the circumstances of the death."

Following April's murder, the Joneses have campaigned for child abuse images to be taken off the internet. They have met Sara Payne, whose eight-year-old daughter Sarah was killed by paedophile Roy Whiting in West Sussex in 2000. It emerged that Whiting had already been on the sex offenders' register for attacking another eight-year-old.

Sara began a campaign for "Sarah's Law", allowing concerned parents or grandparents to contact police to find out if an adult who has contact with a child has a history of sexual offences. The rule came into effect in England and Wales in 2011.

Several other parents of murdered children have set up projects in their name. Denise Fergus, the mother of James Bulger, the two-year-old abducted and murdered by two older boys in Liverpool in 1993, set up the James Bulger Memorial Trust in 2011. It provides holidays and other rewards to young people who have suffered bereavement or been the victims of crime, hatred or bullying, and their families. It makes the same offer to those who have shown "exemplary conduct" or made "a positive contribution" to others' lives.

"I know how much hurt and pain is caused through losing a child," says Denise, "and after a very dark time in my life I decided to try to do some good in my son's name." She adds that her aim is to "help other families and their children who have or are going through a tragic time in their lives".