Reid Scanlon had spent the day with a childminder in Cardiff, when he started to feel pain in his stomach after the incident, his mother Alison Scanlon told Sky News.

She described him as a "typical little boy" with "no fear".

"He decides to throw himself down the bank and throw down," Ms Scanlon added.

The pain persisted and Reid was taken for tests at the hospital.

"They just broke the news that they'd found a mass on Reid's left kidney, quite big in size, Ms Scanlon, from Tonteg in Pontypridd, said. "They were quite worried about it."

"They came back in then to say they'd be transferring Reid to the Noah's Ark hospital to the Rainbow ward. The rainbow ward is the children's cancer ward."

Ms Scanlon said she "can't explain" the way she felt.

"We just looked at Reid and we just couldn't believe it. It didn't sink in, we were just crying and so upset," she said.

Reid was diagnosed with Wilms' tumour, a type of kidney cancer.

According to the charity Cancer Research UK, around 80 children are diagnosed with Wilms' tumour every year.

NHS inform, Scotland's national health information service, says bilateral Wilms' tumour, where both kidneys are affected, occurs in about one in 20 cases.

"Until that day, we thought if it's his left kidney, they can operate, he's still got his right kidney to see him through but that's not the case," his mother said.

Had Reid not fallen over at the park that day, Ms Scanlon is unsure whether his cancer would have been discovered when it was.

"He'd fallen over, and this is the thing, the day before he was doing WWE wrestling with his brother, jumping off the pouffe," she said.

"There was just absolutely no signs at all. Nothing. He was bright, happy, cheerful.

"What they'd said was, where he'd fallen, it caused the mass to bleed and that's why he was in so much pain.

"So if he hadn't have fallen, probably today we'd be none the wiser until a few months down the line it would have started showing out of his stomach."

Ms Scanlon's cousin Jill Desmond set up a GoFundMe page to raise money to support the family during Reid's treatment.

"Overnight she's put it up and it's raised so much money from strangers, from people whose children have gone through the same thing have donated, it's just incredible. We're just so overwhelmed," she said.

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Ms Scanlon thanked all those who had sent messages of support to Reid and donated to the fundraiser "for the kindness, the gesturers, the messages".

"It just blows you away," she added.

"You can't comprehend. One day, he's just a fit and healthy little boy playing and then the following day you get told he's got cancer."