Coco Bradford lost her life after 'several opportunities' were missed by doctors and consultants to save her.
This included failing to order basic blood and stool samples or offering her a simple course of intravenous fluids to rehydrate her.
And her heartbroken mum Rachel Bradford told the coroner, staff then 'unforgivably' tried to cover up their own failings by blaming her daughter's autism and claimed she was being difficult to treat.
Worried parents Rachel and husband Luke initially rushed Coco to hospital on 24 July 2017 but were sent home from the emergency department after being told she likely had gastroenteritis.
But she continued to pass liquid and bloody stools and could not hold down any fluids so was rushed back in less than a day later.
The inquest heard upon admission, doctors still felt her condition was not serious despite the family pleading for more to be done.
Her condition continued to deteriorate and she was transferred from the Treliske Royal Cornwall Hospital three days after admission to the Bristol Children's Hospital where she died of multiple organ failure caused by sepsis.
Mrs Bradford, of St Ives, Cornwall, told the coroner on Monday she has fought for four years ever since to expose the truth and failings around her daughter's death at Treliske.
Fighting back tears, she said they discovered medical staff had "multiple opportunities to change the course of Coco's treatment that could have saved her life."
This was from the first trip to the emergency department right up until her transfer to ITU.
She added: "None were acted upon. We do not accept human error should be a factor. This was failing of the most basic level of care. Every member of staff denied knowledge of bloody vomit and faeces yet it is in all notes. There were notes blaming Coco for being uncooperative. She was in a terrible amount of pain and dying in front of their eyes."
The pathologist told the coroner Coco died on 31 July 2017 following withdrawal of intensive care and multi organ failure. She suffered hemolytic uremic syndrome as a result of e coli 157.
The inquest heard she had been transferred to Bristol Children's Hospital from Cornwall three days earlier.
Rachel said the failures ranged from not ordering blood samples, giving her a pain score of 0, not picking up signs of sepsis and being not given an IV drip to rehydrate.
She added: "We have never been given the full details of the clinical picture and I'd like you to spare a thought how her sister Chelsea is feeling. A fellow clinician telling her not to worry and her little sister is fine. All concerns and questions around sepsis and e-coli dismissed - when actually she was dying.
"Had we not persisted and if Chelsea was not a healthcare professional, we would never know the truth of what happened to Coco. We are still waiting for the full truth l but I suspect that may never happen.
Rachel added that she found all the failures, delays and lack of treatment "staggering in this day of modern medicine."
She said: "Luke and I became more and more upset and concerned by the air of nonchalance of staff around her condition. It seemed they had no idea what was wrong with her. They did not recognise she was close to death.
"We were told she'd be fine and every time we asked we were told a stool sample was not needed or been requested. I felt they always thought I was overreacting and worrying for nothing every time we asked questions. Blaming Coco and her autism for their own incompetency and failure to treat her is absolutely unforgivable."
Rachel said she had been told that e-coli 157 - if treated with aggressive fluid rehydration - had a survival rate of 97 per cent with babies and the elderly those who suffer worse.
And she told the coroner the effect on her family of their loss had been indescribable.
She added: "To meet Coco was to love her. Her life with ours was getting better and better and she learned to communicate again. A week before she fell ill, I posted about how happy she was. She was in such a good place and we finally felt she would have the life she deserved and was going to mainstream school full time.
"If I could have sat in the bed with her and die as well I would have done. Coco was my absolute world and living without her is almost too much to bare. Coco was a strong and healthy six and a half year old. The effect on our family has been so deep and so divisive.
"Life will never be the same again. We are forced to live a life we did not want to. We do not deserve. Not only have we lost beautiful Coco without the chance to live her life, we have been fractured to a level you will never understand."
She said the family have spent four years battling and being lied to as she tried to get to the truth.
"We have been treated with the upmost contempt," she added. "There have been tears, tears and more tears. Every single day. We just want the truth. You never know how it feels until it happens to you."
Dr Saul Diaz Reales was the doctor who initially saw Coco in the emergency department at Treliske and sent her home.
He told the coroner he had no evidence her blood pressure was not normal.
He added: "I would have wanted to see a blood sample if that was the case. In the benefit of hindsight it is difficult to have a case where Coco should not have been admitted. Looking at the evidence I had at the time I fail to see what I could have done better."
He added he had 'no evidence' of there being blood in her stools and at the time was "happy and satisfied" to send her home.
The inquest at Truro Coroner Court continues.