Most of the errors were unlikely to cause serious harm
Mistakes are being made in a high number of drug treatments given to children in hospital, experts warn.
A snap-shot study by the University of London of five hospitals in the city found 13% of the 3,000 prescriptions they examined had an error.
And a fifth of drugs given to children in these hospitals during 2004 and 2005 were administered incorrectly.
Most errors were harmless but a small number were potentially fatal, Archives of Diseases in Childhood reports.
On five occasions, one of the investigators intervened to prevent the patient suffering the consequences.
Over a period of two weeks, they watched how nurses gave drugs to children on 11 wards at the five hospitals.
They picked up 429 administration errors among the 1,554 doses of medicine given to 265 children, giving an overall error rate of 19%.
When pharmacists reviewed the drug charts of 444 children treated in the hospitals over the fortnight, they found and corrected errors in 13% of almost 3,000 prescriptions. The majority were incomplete prescriptions, but a third were dosing errors.
Although the study involved only five London hospitals, the authors believe the results would be similar in other UK hospitals.
And despite the study being carried out five years ago, the researchers say the findings still stand today.
Co-author of the study, Professor Ian Wong of The University of London, said: "It is highly unlikely that the situation has changed since our study was done.
"That is because prescribing for children is very difficult."
Most drugs are formulated for adults not children, meaning doctors have to make their own dose calculations based on the child's age, weight and clinical condition.
Many drugs given to children are used unlicensed, meaning they have not been tested and approved for use on children, which compounds the problem.
Prof Wong said it was important to look at ways to minimise the risk of errors.
"It is a challenge working on the ward and humans are bound to make errors. Most would not cause serious harm, but some are potentially fatal."
He said the UK was developing ways to cut the risk of drug errors, including using electronic prescribing systems that flag up mistakes.
Dr William Van't Hoff, of the Royal College of Paediatrics and Child Health, said the high error rate reflected the complexity of treating children, but was still very concerning.
"There is a need for more research into the causes of these errors and training to prevent mistakes being made."
The National Patient Safety Association said it had recently looked at patient safety among children in hospital and had identified action points for NHS organisations, which include reviewing local standard operating procedures for medicine management.
Liberal Democrat shadow health secretary Norman Lamb said the results were a sign of the government's failure to improve safety in the NHS.
Tory shadow health secretary Andrew Lansley said the Conservatives would improve the data collection of errors in the NHS and incentivise year on year improvements in avoidable incidences.
A Department of Health spokeswoman said patient safety was the highest priority for the NHS and the government.