Millions are to be spent on research to allow adult medicines be given more safely to children.
Currently many sick children and babies are given lower doses of adult medicines, which may increase the risk of side effects.
The World Health Organisation and Unicef will spend a grant of almost $10m from the Gates Foundation to carry out more research.
The programme aims to develop better rehydration drugs and antibiotics.
Carrying out clinical trials of medicines in children is both difficult, time-consuming and expensive, and many new drugs are licensed only for adult use.
This means that clinicians must take personal responsibility for calculating the dose needed for a smaller body, and prescribe it outside the terms of the licence.
This can not only make the prescribing of drugs more complicated, but is no guarantee of safety, as children can react differently to medicines, even if the dose is "correct" for their weight.
An additional problem is that some medicines are produced only in tablet form, making the job of preparing them for small children even more difficult.
It is estimated that more than 50% of medicines prescribed for children have either been developed for adults, or not properly tested beforehand.
Carissa Etienne, the WHO's Assistant Director-General said: "We must take the guess-work out of medicines for children.
"Children are suffering and dying from diseases we can treat, and yet we lack the critical evidence needed to deliver appropriate, effective, affordable medicines that might save them."
The grant, from the charitable foundation set up by Microsoft's Bill Gates and his wife Melinda, will be used to produce guidelines on dosing for existing paediatric medicines, and to advise on testing, treatment and the use of medicines in such patients.
A key target are the treatments given to millions of children dangerously dehydrated after bouts of diarrhoeal disease.
The combination of oral rehydration salts and zinc is not only unpleasant tasting and difficult to administer, but pharmacists have to work out the correct dose for each child.
Another aim is to make the antibiotic amoxycillin more easily available in a child's dose in cases of pneumonia or other bacterial infections.
Professor Peter Helms, a child health specialist at Aberdeen University, said that similar issues were found in the UK, with doctors occasionally applying a combination of "guess-work and good judgement" to determine the dose of drugs.
He said: "Babies are not little adults - for a start they are about 80% water, compared to 70% in adults, and their internal organs are not as mature and may find it harder to detoxify."
However, he said matters were improving, particularly following the publication of a "Children's British National Formulary" - a written guide to drugs along the same lines as the version available for many years for adults.