Children with cancer could be missing out on potentially life-saving drugs because EU regulations allow drug companies to opt out of running trials in children, cancer experts say.
Since 2012, 62% of approved EU cancer drugs were not tested on under-18s.
But cancer scientists say the rules should be changed so that adult cancer drugs have to be tested on children.
They are also calling for the age limit for adult clinical trials to be lowered to include teenagers.
Who is saying all this?
The Institute of Cancer Research in London, cancer hospital the Royal Marsden and parents of children who have died from cancer say nowhere near enough cancer medicines are being trialled in children or licensed for use in children.
A consultation is currently running on the future of EU rules on clinical trials, which means there is a chance they could be changed.
What happened to Grace?
Grace Kelly was four years old when she died from a very aggressive tumour in her kidney, just three weeks after she had been diagnosed, in 2014.
Her mother, Jennifer, who is a GP, believes the regulations should be tightened to allow more tests of cancer drugs on children to take place.
"There could have been a treatment for her out there, but we were told at her diagnosis that her cancer was [terminal]," she says.
She says huge progress could be made in treating rare cancers if the rules were changed.
"They are not looking at the children right now," she says.
The Grace Kelly Ladybird Trust has been set up in her memory to push for better treatments for children with cancer.
Why can drug companies opt out of trials in children?
The rules, called the EU Paediatric Regulation, which were set up in 2007, allow drug companies to be granted a waiver by the European Medicines Agency (EMA) when it comes to trialling cancer drugs on children.
This usually happens if the drug in question targets a cancer such as lung cancer that does not affect or is very rare in children.
But cancer researchers and charities say the way a drug works may mean it could be effective in children - even if it does not seem to be relevant.
Cancer medicines are changing, and they are often designed to treat many different types of cancer, which may have the same genetic target, rather than just one.
So they want fewer waivers to be issued and more trials on children to take place.
Why do drug companies want to avoid carrying out trials in children?
It is not that they do not want to carry out the trials and help cure childhood cancers - it is more that they want to make sure the trials are worthwhile.
Not all cancer drugs will be relevant or safe for children.
Creating cancer drugs is an expensive process and developing treatments for children, in whom cancers are rare, is a financial challenge for many manufacturers.
That is why it has been suggested that greater economic incentives are needed for companies to run trials in children.
Are drug trials on children really complex?
Doctors have to be very careful when treating children with cancer, of course.
They will always follow up closely after drugs are given - but children with cancer are capable of being treated just like adults, often with the same drug doses.
What difference could tightening the rules make?
Prof Paul Workman, from the Institute of Cancer Research, says children are missing out on innovative cancer treatments now common in adults.
As a result, he wants the regulations to be updated so that children with cancer - even if it is rare or terminal - get the best possible treatment with the latest medicines.
Prof Louis Chesler, honorary consultant in paediatric oncology at the Royal Marsden NHS Foundation Trusts, says access to cancer medicines has improved in recent years and more cancer drugs are being developed - but there are still improvements that could be made.
"Is there any reason why adult cancer trials can't lower their age limits to 16, or even 12?" he asks.
And he adds: "Everyone realises this is a challenge, but more incentives are needed for drug companies to make a return and to speed up the development of drugs."