Research from the charity Clic Sargent found that some end up in debt and are forced to live on credit cards to ensure their child can attend hospital.
On average, families travel an extra 440 miles for treatment, with 8% travelling 1,000 miles or more, the charity said.
Rachel Holt spent £700 a month on petrol to take her 11-year-old son Reece for treatment and needed help from charities to survive.
Reece, from Overton in Lancashire, was diagnosed with a brain tumour in May 2016.
He was transferred to Alder Hey Children's Hospital in Liverpool, a 130-mile round-trip away from his home.
He was also treated at Clatterbridge Cancer Centre on the Wirral, a 180-mile round-trip.
Treatment took place five days a week for six weeks, and the family paid out £30 to £40 a day on their credit card for petrol.
Mrs Holt said: "Financially the extra costs we've had during Reece's treatment have been difficult.
"Before this happened we had no comprehension of how much it would all cost.
"During Reece's treatment, we spent every penny of our savings, borrowed from our family, and our credit card balance went from zero to £1,600 in six months.
"I'd never used our credit card before, but without it I would have struggled to even get Reece to hospital for treatment."
According to Clic Sargent, just 6% of parents receive help from the NHS's Healthcare Travel Costs Scheme (HTCS), which can help with travel costs for families with incomes up to £16,000.
Some 78% of people surveyed in a poll of 106 parents for Clic Sargent were unaware the scheme existed.
This is despite more than 40% saying that assistance from the Government with travel expenses could help them better cope financially.
Clare Laxton, assistant director of policy and influencing at Clic Sargent, said: "Parents are routinely travelling hundreds of miles for their child's cancer treatment every month, and despite the huge financial burden this causes, tell us they are left largely unsupported by existing Government schemes.
"With a large proportion of families we asked not knowing about the scheme and many families not eligible for some assistance, there is a clear need for governments across the UK to review the travel assistance schemes that currently exist and whether they are getting to the people that really need them."
Clic Sargent has a petition campaigning against cancer costs at www.clicsargent.org.uk/cancercosts