Ayaan Hussain, who was almost two, started vomiting in October last year and increasingly struggled to balance.
His mum Claire Kennedy said she sought medical help for her son multiple times in November and December 2014. Each time she claims she was told by doctors that the toddler may be suffering from a stomach bug or intolerance to dairy.
On one occasion she said an "uninterested" medic ignored her pleas for more tests and said: "I cannot possibly arrange a test for every poorly child who comes through this waiting room."
Ten days later Ayaan, from Bristol, suffered a fit and a respiratory arrest. When he got to hospital a brain scan revealed he had a brain tumour and he died soon after.
Describing the moment Ayaan was rushed to hospital after suffering respiratory arrest, Kennedy said: "It was terrifying, I didn't know what to do. His little body was rigid. I could tell his heart was racing.
"I had been forced to nurse my own son in the final days of his life without any medical help."
A scan revealed the toddler had a brain tumour and he underwent an emergency operation, but it was unsuccessful and Kennedy was told nothing more could be done.
The next day, 11 December, Ayaan died from swelling on the brain. He was in his mother's arms when he died.
The coroner ruled there was a "missed opportunity" to diagnose the tumour and she "considers this to be a failure".
An investigation into the toddler's care concluded he would have had a 40 to 80% chance of survival for up to five years if his tumour had been detected earlier.
Kennedy said she took her son to local GPs three times and made two separate visits to accident and emergency between November and December 2014.
In a statement read out in court Kennedy described her son's behaviour when he first became ill: "He was getting more and more unwell before my eyes. His behaviour was now erratic and he was having tantrums that he had never had before.
"Sometimes he would go in to fits of crying and screaming and would often seem confused."
During his visit to Bristol Royal Hospital For Children Ayaan was treated by doctor Isabell Hancock, a registrar who had been working in the department for three months.
Dr Hancock told the court she had neurological concerns due to Ayaan's persistent vomiting, but his behaviour and medical checks were normal.
She sought advice from a senior consultant who diagnosed the toddler as having problems digesting dairy, but a paediatrician referral to Ayaan's GP was sent to the wrong address.
She said she wished she had flagged up neurological concerns on his notes, but added she couldn't check the toddler's eyes better for evidence of a brain tumour because he was "hyper".
Kennedy brought Ayaan back to the emergency department on 30 November, after he started holding his ear and arching his back.
She saw consultant paediatrician doctor Nicholas Sargant, who told the inquest it was the busiest day in the emergency department that year.
Speaking at the hearing in Flax Bourton, near Bristol, Kennedy said: "Dr Sargant seemed agitated and uninterested once Ayaan failed to cooperate.
"I believe he didn't listen carefully to all of my concerns which I felt led to the misdiagnosis.
"I begged him to undertake some checks on my son.
"Dr Sargant replied - and I will never forget his words - 'I cannot possibly arrange a test for every poorly child which comes through this waiting room'."
Dr Sargant told the court Ayaan was "tired and irritable" so he couldn't complete all of the checks - including a check to observe the back of his eyes - and diagnosed a stomach complaint perhaps caused by lactose intolerance.
He added: "Whilst we are all devastated that we missed Ayaan's tumour and missed an opportunity probably to save his life – although not certainly – it is a very rare diagnosis.
"What I am most disappointed about is, in terms of seniority, I was the most senior person to have seen Ayaan prior to him presenting in his final admission and this is very hard to live with.
"I have never had anything like this happen to me before and if the story had been presented to me as I have heard it today I have no doubt I would have ordered a scan."
Senior coroner for Avon, Maria Voisin said she considered it a "failure" of the University Hospitals Bristol NHS Foundation Trust that signs of the little boy's illness were not picked up sooner.
Recording a narrative conclusion at Avon Coroners' Court she said she considered the second emergency department visit as a "missed opportunity" to diagnose the tumour and that she "considers this to be a failure."
She added that "on the balance of probability" diagnosis on the visit "would have resulted in treatment which would have prolonged his [Ayaan's] life."
Speaking after the case Kennedy said she was pleased with the conclusion and added: "I hope today prevents this from happening again to anyone else."
Since the toddler's death the trust has bought a new piece of equipment to photograph the back of children's eyes to check for signs of tumours.
In a statement released after the conclusion, Deborah Lee, deputy chief executive of University Hospitals Bristol NHS Foundation Trust, said: "We are deeply sorry for Ayaan's death and our thoughts are with his family.
"The coroner has found that there was a missed opportunity to take action following his second visit to our emergency department.
"This was acknowledged through our own investigation, which we did after Ayaan's death, as a way of ensuring that any possible learning was identified.
"As a result, we have implemented additional guidelines for clinicians to minimise the risk of a similar event ever occurring in the future."