Children will have access to improved treatment following changes to European laws, campaigners say.
The law will aid research into children's cancer treatment
From this week, any new medicine licensed in Europe must be examined for its potential use for children.
The changes will apply to all kinds of drugs, but cancer experts say they will be of particular benefit to children with the disease.
Cancer Research UK says the law will allow more knowledge about how anti-cancer drugs work in children.
The charity says such research is currently affected by drug company concerns over the challenges of developing medicines for children.
Around 2,000 people under the age of 19 are diagnosed with cancer in the UK each year.
Children with a wide range of conditions are currently given scaled-down doses of medication designed for adults which may not have gone through full clinical trials.
This means doctors often have to estimate the dose which a child will need, increasing the risk of dangerous side-effects or ineffective treatment.
The Association of the British Pharmaceutical Industry (ABPI) estimates 90% of children in neonatal intensive care units are given unlicensed medicines, as are 45% of medicines used on general children's wards and up to 20% of drugs prescribed to children by GPs.
Drug company benefits
Under the new legislation, there will have to be a paediatric investigation plan which sets out which age groups will need to be studied before a drug can be made available to patients.
The law makes distinctions between the different needs of newborns (nought to one-month-old), infants (one to 23 months), young children (two to 12 years) and adolescents (aged 12-18).
Applications could be made for medicines which would clearly not be used on children, such as hormone replacement therapy, to be exempt from the law.
The changes apply to both new and existing medicines.
Pharmaceutical companies will be rewarded for putting extra efforts into researching children's medicines with a six-month patent extension on new medicines and exclusivity on data on medicines which are out of patents for 10 years.
Dr Sally Burtles, director of Cancer Research UK's drug development office, said: "Childhood cancers are very different from adult tumours.
"This new law presents a massive boost to drug discovery programmes across Europe and will encourage further collaboration between pharmaceutical companies and childhood cancer experts."
Dr Richard Tiner, medical director of the ABPI, said: "This new legislation represents a huge opportunity for European research into children's medicines and the UK is probably the most prepared country in the EU to take this forward.
"Clinical trials obviously raise questions of ethics, plus each stage of a child's development from infant to teenager requires different formulations.
"As a result, patient numbers per drug may be small - especially for rare conditions.
"In the past this made it difficult for drug companies to recoup the massive costs of research."