Imagine sitting opposite child abusers cackling about the evil crimes they have carried out on innocent boys and girls. 

Then contemplate trying to show compassion to those same people after hearing their tiny, defenceless victims relive and recount the horror of their abuse.

This is the reality of being a child protection officer - a specialist role that Sharon Birch carried out for eight of her 20 years in the police.

After years of dedicated service Sharon, who rose through the ranks after starting out as a trainee detective, has been left with horror movie flashbacks after witnessing the worst kinds of evil.

The veteran officer has told The Crime Desk why she was forced to hold back her feelings of disgust and anger to show them 'compassion' and even 'sympathy'. 

'You have to shut something off in your head when you interview these people,' she explains. 'You're not going to get anybody to talk to you if you don't treat them decently and like a human being.

'You have to detach from the emotion and appeal to their compassion.' 

But she clarifies: 'I do not personally have compassion for them, but I deal with them in a compassionate way. 

'I'm not going to be openly judgmental to them. Whatever I might feel inside, I keep inside.

'You get people that feel sorry for themselves and blame everybody else and just cry - and you can have a degree of sympathy with some of them. But when you boil it down to what they've done, that soon disappears.'

One child killer that haunts her to this day is a mother who 'just laughed all the way through' the interview. Another that 'chilled' her was a mother who had strangled her son to death in a fit of rage. 

But there is a particularly harrowing child death that will never leave her. 

Sharon gets emotional as she recalls the death of an eight-month-old baby, who had been admitted to hospital on two previous occasions. The infant choked to death after a bottle was forced into her mouth.

Her father was found responsible for her death in a coroner's court. However, police could not secure a criminal conviction because the Crown Prosecution Service did not bring charges due to insufficient evidence.

'She was only eight months old and that was tragic,' Sharon says. 'I still have flashbacks of her randomly when I am in the shower or just going about my business. 

'You can be totally switched off and suddenly you get that flashback and they never leave you.

'The warning signs were there. We had strategy meetings with the agencies beforehand. There were signs there that he might be hurting the baby.

'I remember sitting in the strategy meeting saying unless we can do something, he might end up murdering this baby. And then a month later, she was admitted to hospital and she died.'

Aged just 19, Sharon moved from Hartlepool to London to launch her career with the Metropolitan Police in 1985. The former detective spent her early years working on robberies, assaults and grisly murder cases across east London and the West End in the 80s and 90s.

She recalls a man who had been dragged 100ft underneath a bus, where his skin was 'ripped off, burnt and detached'. And another where she did a welfare check on a man, only to find his lifeless body 'bloated, split and oozing'.

'I can smell it now just thinking of it,' she says, explaining that he had been left in his flat for two weeks with his gas fire on full blast. 

But it was her move up north to Cleveland Police in 1997 which plunged her into the murky world of child abusers and baby killers when she became a child protection officer.

By this point, she was a mother to three young children.  

'I'd go home and I'd hug my children extra tight,' Sharon, now 60, remembers after spending a day at a child post-mortem, interviewing child abuse victims and confronting their abusers.

'When you talk to these very young, vulnerable children, they are having to relive and recount the worst things that have ever happened to them in their life. You're a stranger and you have to build their confidence and trust so they will talk to you.'

She says you then 'use different parts of your brain' to interview their alleged abusers. 

'What I do know about sex offenders is very often they want to stop, but don't know how to, and it escalates and escalates, and if they could just stop I think a lot of them would,' she explains.

'If you are hostile or you don't communicate properly, or they know that you're scorning them or looking down on them, they're not going to talk to you.

'There's nothing wrong with being reasonable with somebody. Communication is key in these circumstances.

'If you detach from the emotion and ask them and appeal to their compassion, I suppose.

'Some are just pathetic but some are really quite calculating and there's a huge degree of manipulation that goes into abusing children, because there's the grooming beforehand.'

Sharon says that child abuse is a crime 'that stretches across all of society from the aristocracy right to the most vulnerable, deprived people'. 'It is everywhere,' she adds.

The hardest part of her work, Sharon insists, is 'without a shadow of a doubt the child deaths and post-mortems'.

'It's not natural for children to die and although it does happen regularly, it carries an emotion with it. It's different to the other things.

'When you see a baby on the slab and they've got bruising on the bottom of the feet and they can't yet walk and you know that somebody's hurt them, it's things like that you need to look for and that the pathologist looks for. It's the evidence factor.

'You've got to stop thinking that's a baby, that's somebody's child. What can we do that's best for that person?'

Putting the emotion aside, Sharon says post-mortems, for both children and adults, as 'very respectful'.

'I dealt with it by knowing that I was there for a reason, that it was part of my job,' she explains.

'We were there to get the best information that we could to establish what had happened to give some resolution to the families, whether that was a sudden death,  a tragic accident or unfortunately at the hands of somebody else.'

That does not, however, take away from the smells and the sounds of being stood in the mortuary.

'The first time you're like a rabbit in the headlights,' Sharon adds. 'You do get used to that process. You do get used to the smells. But you never lose that first time you walk in and the dead body.

'You know it's a person - or has been a person - and from that moment in, the professionalism kicks in.

'You're watching the pathologist dissect the body... you can see the injury.  

'I think the longest I was there was for five hours and it's hard. You stand in there and it's the smells and the sounds. It's those sensory things that you remember more than anything else really.

'There's a circular saw that removes top of the skull so you can take the brain out, the cutting of the stomach, the smell of the stomach contents, all of those things.'

But perhaps even more distressing, Sharon says, is having to remove a newborn baby from a drug-addicted mother.

'The keening cry of a drug-addicted baby is one of the most haunting things I've ever heard,' she says.

'We would have to remove the baby and take it into foster care. And then you've got the howling of the mum as well, that's awful. That's a really tough job.'

Despite the trauma of working with victims of child cruelty and confronting abusers, Sharon says she misses her job.

'It's such important work and it's so vital,' she says. 'I do miss it, although I would never want to go to another child's post-mortem.'