Dylan Crean was brutally murdered by a man the tot trusted and loved - as his desperate mother battled in vain to save his life.
The tot was killed by Katie's partner Peter-Gavin Cawser, 22, after she moved from her hometown of Birmingham to his home in rural Derbyshire because she thought it would be safer for Dylan.
Little did she know that the move would end in tragedy.
She had met Cawser while clubbing with a friend at the Custard Factory in Digbeth not long after splitting up with Dylan's dad.
"We started talking and got together six months later," recalls Katie, 23 and from Sheldon.
"The area where I was living in Birmingham was quite rough so I decided to move to Derbyshire to be with Gav, as I called him, as that was his home.
"I thought it would be safer for Dylan.
"We lived with Gav's mum and were happy, we had a nice relationship. Gav was loving and caring.
"Dylan loved him, and Dylan was very definite about who he did and didn't like."
On August 26, 2011, the couple moved into a house together in Albert Village, Swadlincote.
Everything was going well for them. Dylan was looking forward to his fourth birthday and was excited to be starting nursery in a couple of weeks.
But, four days after the move, Katie's world turned upside down.
Cawser returned from his job in house clearance and became incensed over a box of Dylan's toys left in the couple's bedroom.
"Gav came back from work and everything was fine," says Katie.
"Then something in him just switched, it was like he became a different person.
"He started beating Dylan up. I was screaming, trying to stop him.
"I yelled 'Stop! Get off him! Leave him alone!' but nothing worked.
"I saw him strike a blow at Dylan's head. It was awful.
"I tried to pull him off, I got Dylan away from him twice but he carried on beating us both up then he threw me down the stairs.
"It was all over in about 20 minutes and I knew then that something terrible had happened to Dylan, the worst possible result there could have been."
Katie said that Cawser refused to allow her to call an ambulance, despite it being clear that Dylan was seriously injured.
Instead, he tried to convince Katie to tell the authorities that she was responsible for her son's injuries.
It was 20 minutes before Cawser left and she could alert the emergency services.
"Gav wouldn't let me call an ambulance, I sat with Dylan, trying to keep him talking," she says. "But then he started fitting. "Gav went off and I called 999. I could barely speak because I was crying so much.
"He came back just as the police and ambulance arrived."
Dylan was blue-lighted to Derby Royal Infirmary then transferred to intensive care at Queen's Medical Centre in Nottingham.
The police arrested Cawser.
Katie gave her statement to the police and went to be with her son in hospital.
"My little boy died in my arms the next day from his head injuries," she says.
"That night, I just felt numb.
"It was 11 days before Dylan's fourth birthday, so I bought birthday presents, teddies and things, to the hospital for him.
"I moved in with friends, moving from one friend's house to another. I couldn't seem to be comfortable staying in one place."
Katie says her mum helped her to organise Dylan's funeral.
"It was lovely. We had a horse and carriage for him because he loved horses," she adds
"We only invited those who were close to him, we didn't want it to be a circus. His favourite colours were blue, green, yellow, orange and red so we got people to wear coloured T-shirts.
"He hated the colour black - he used to hate it when I painted my nails black. We buried him with his teddies, the birthday presents I'd bought him in hospital.
"Then my friends took me out and got me drunk. I didn't really speak to anyone."
The case took more than a year to come to trial.
Cawser refused to plead guilty right up until the last moment, instead trying to lay the blame on Katie.
She says: "It was only when it came to trial that it started to sink in.
"That year felt like a blur.
"The post-mortem showed Gav had inflicted more than 70 injuries on Dylan. He had punched and kicked him and thrown him against the wall.
"Gav sometimes took medication to help him sleep but that was checked and he was clear of mind when it happened.
"I don't know what made him switch, I suppose I'll never know."
Cawser was jailed for life in September 2012 and ordered to serve a minimum of 18 years before being considered for parole.
Judge Michael Pert QC said it was 'vindictive and uncontrollable rage of an adult man against a three-year-old child'.
A paediatrician told the court that he had never seen such a sustained physical attack on a child.
Katie has been trying to pick up the pieces of her life, but her grief is still very raw.
"I've been put on antidepressants and had counselling," explains Katie, who is not currently working.
"I've moved back to Birmingham and got in touch with my first boyfriend, Richard. We'd been together when I was about 14 and, in time, we've got together again.
"Richard has helped me a lot, so has my family and his family.
"They're encouraging me to keep taking my driving lessons as that would give me some independence."
Katie tries to remember all the good times she had with her son.
"When I think of Dylan, which is all the time, I try to think of him when he was two, because that was before we met Gav.
"Dylan was cheeky and funny, a real character.
"He gave the people he loved pet names. He was very bright, he knew his alphabet before he was two.
"He was my only son, I only wanted one.
"We'd never taken Dylan on holiday because I was paranoid he would get sunburned. He had ginger hair and really fair skin.
"It meant I'd never even taken him to the beach. I'd planned to take him to Spain this year."
Katie has begun working with the NSPCC and is putting on an charity event with Sarah Edwards, whose partner also killed her son.
Sarah was wrongly blamed for the death of Kamran Rose, aged 18 months.
Katie says: "Sarah lost her son six years ago. She has been a really big help to me. Whenever I feel a bit down, she's always there for me to talk to.
"She's since got married and had two beautiful daughters.
"She's such an inspiration. I hope maybe, in time, that will happen for me.
"I feel that if our charity night helps to save just one little boy or girl then it will be worth it."
Social Services and the police have launched a Serious Case Review into Dylan's death.
A spokeswoman for Birmingham City Council said: "Birmingham Safeguarding Children Board is in liaison with Derbyshire Safeguarding Children Board and the police in relation to the death of three-year-old Dylan Crean.
"The Birmingham board is liaising with Derbyshire because the mother and the deceased child previously lived in Birmingham.
"We can confirm he was known to a number of agencies (including children's social care) however it would be inappropriate to comment further as there will be a serious case review into his death."
* Katie and Sarah are holding an NSPCC charity disco and auction at the Emerald Social Club, Green Lane, Small Heath, on May 10. To book tickets, call the NSPCC Midlands Fundraising Team on 0844 892 0217
1,300 kids known to be at risk in Birmingham
Local authority statistics show that last year 1,307 children in Birmingham were subject to a child protection plan because they were considered to be at risk of abuse, including 576 who were aged four or under.
Research shows far more cases go unreported and many children suffer in silence.
The NSPCC Birmingham Service Centre in Newhall Square helps make a real difference to Midland children most at risk, including those affected by neglect or sexual abuse, vulnerable minority ethnic children and young people in care.
Deborah Chandos-Hall, head of Midlands fund-raising says: "We wouldn't be able to continue to run our vital services supporting children and families across the West Midlands without the help of local people who feel as passionately as we do about protecting children from harm.
"If anyone would like to learn more about getting involved with the charity and volunteering with us, please do contact me on 0844 892 0217.
"I encourage people in Birmingham to show their support by supporting the charity event on May 10."