Hers is one of the most harrowing yet tender stories I have ever heard. On October 21, 1966, cadet nurse Dianne Thomas answered the call for help at Aberfan and was given a task that haunts her still.

It also explains why she did not return until today, joining the King, Queen and Prince of Wales to mark the 60th anniversary of one of the darkest days in post-war history.

Dianne was sent to the makeshift mortuary in a local chapel to wash some of the bodies killed by the 150,000-ton wall of mud and slurry from the coal tip that engulfed Pantglas Junior School and neighbouring houses. Of 144 who died, 116 were children.

If grieving parents were to have to identify their children, no one wanted their last glimpse to be any more horrific than need be. Dianne had only just turned 18.

'You didn't want the parents to see them with...' and then she asked me not to repeat the more graphic details. 'Let's just say I washed them with warm soapy water.'

There was one eight-year-old boy she can never forget. 'I was going to be the last person to hold him like his Mam had held him. So I held him in my arms and started singing to him. 'Hey, diddle, diddle...'.' It was hard enough to listen to this story, how much harder it was for her to retell it.

'And then I put my hand in his pocket,' Dianne went on. 'There was his spelling book. And he had just scored eight out of ten in his test. And I remember thinking that if I could only find his Mam, it might offer her the tiniest comfort if I could tell her that.'

In the mayhem, she never learned his name. She was working on one side of a sheet in the emergency mortuary. On the other, the chief mortician was preparing bodies for new coffins 'piled floor to ceiling'.

The very smell of fresh pine, said Dianne, was still enough to trigger flashbacks. 'And whenever I hear 'Hey, diddle, diddle', I well up,' she added. From that day, she lost her ambition to be a nurse and pursued a career in child protection, becoming a social worker and latterly a schools inspector.

Such were the memories on Wednesday. The King, who has been to Aberfan many times and has forged a long-standing friendship with a bereavement group called the Aberfan Wives, was never going to miss this. He was here in 2016 for the 50th, as Prince of Wales, representing the Queen.

In 1966, she came eight days after the disaster and broke down in the home of a mother who lost two children. She would return to this one tiny Welsh village more often than some major cities.

On Wednesday, the King was joined by Queen Camilla and Prince William. All laid floral tributes at the Garden of Remembrance on the site of the school before a walkabout to the community centre.

This level of royal representation is normally reserved for major state occasions. But then it's hard to think of a date that is seared into the Welsh collective memory as deeply as this one. Every level of the Welsh state, from First Minister down, was on parade.

The day began with a private ceremony up at the steep cemetery where the children lie side by side. It was a rich carpet of fresh flowers. Desperately poignant was the wording on several graves: 'Together at last'. Sixty years on, many bereft parents have died – and almost all have chosen to be interred alongside the son or daughter who never returned. In some cases it was both.

They include Cliff and Iris Minett who lost Marylyn, ten, and Carl, seven, that day. 'I did have a bit of a breakdown this morning at the cemetery,' said their surviving child, Gaynor Madgwick. 'I normally try to contain myself, but that minute's silence got to me. My father and my mother and my brother and my sister, they're all together. I couldn't breathe.'

She recalled how her little brother hated going to school. 'Mum only managed to get Carl there that day because it was about to be half term and she told him he'd be home by noon. She always had to live with that guilt.'

Gaynor only escaped because she was shielded by a radiator. She has devoted much of her life to counselling others. Visit St Fagans National Museum of History and you can see some of the things she has donated for posterity, including Carl's football, Marylyn's brooch and her own handwritten childhood account of the disaster, called A Sad Story.

On Wednesday, she was in charge of escorting Queen Camilla at the local community centre.

Prince William joined Denise Morgan, who lost her 11-year-old sister, Annette. 'We did everything together,' she told me. 'We even slept in the same bed.' She recalls her father returning from the morgue, clutching Annette's purse and saying she looked 'so peaceful – as if she was sleeping'.

That is precisely why the delicate work of Dianne Thomas and her colleagues was so important.

Doing the introductions for the King was Jeff Edwards, the last child to be pulled from the wreckage – just after 11am. 'From our darkest hour,' he told the monarch in his speech of welcome, 'to this present day, you have remembered us, you have stood with us.'

The King told them: 'We have heard stories of heartbreak, but also of heroism. We have heard of the most agonising grief but also of the most inspiring grace. Hearts far from this valley will ache with yours.'

He then met people like David Wilson, who at 23 drove seven nurses up from Port Talbot in his roofer's van. He promptly joined miners digging into the quagmire only to unearth the body of a man trapped under a boulder.

Here, too, was Mair Morgan, 85, the last surviving teacher. She managed to get all the children in her reception class, at the far side of the school, out safely. She stayed here for the rest of her career, rebuilding a shell-shocked school from a fleet of caravans.

The royal party were serenaded by the Ynysowen Male Voice Choir, set up in the aftermath as a form of therapy for grieving menfolk. They include Ray Lee, who continued to cling on to hope until 10pm that dreadful night when he identified his only child, Anne, in the Bethania Chapel. 'I like to think I'm singing to her when we sing,' he said.

As the royal convoy drove away and the elderly witnesses to the unimaginable drifted home, I heard an enchanting sound from the memorial garden. For it is also now home to a playground.

Here were the same happy shrieks you might have heard before 9.13am on October 21, 1966. Even in Aberfan, life goes on.