More than one in five parents would not tell their GP if their child suffered a reaction to a medicine, while almost six in 10 would fail to tell a pharmacist, according to a poll.
The survey also found many are unaware they can report side-effects via a special "yellow card" scheme run by the Medicines and Healthcare products Regulatory Agency (MHRA).
Most medicines on the market are not tested on children although regulations introduced in 2006 say drug developers must submit plans for clinically investigating new drugs in youngsters.
Today's MHRA survey of 9,142 people found 78% would tell their GP if their child suffered a suspected reaction to a medicine, but 22% would not.
Parents in London and the South East were the least likely to report problems to their GP, with 69% in London and 72% in the South East saying they would.
For pharmacists, the UK average was 44% of people saying they would tell them, with London and the South being the lowest reporters at 39%.
But while Londoners were the least likely to report problems to health professionals, they were more likely to report any side-effects to the MHRA's yellow card scheme, with 15% of London parents recognising the scheme compared to a national average of 9%.
Research has suggested that only 10 to 15% of serious side-effects are reported in the entire population, with the rate in children being about half of that figure.
June Raine, director of the MHRA's vigilance and risk management of medicines division, said: "It is vital that parents and carers as well as healthcare professionals use the yellow card scheme to report any suspected side-effects.
"Identifying side-effects in children can be more difficult and it may only be a parent or carer who is able to identify symptoms in a child that they cannot explain themselves.
"Parents and carers have an important contribution to make identifying potential safety problems and so helping the MHRA protect public health."
Dr Hilary Cass, president of the Royal College of Paediatrics and Child Health, said: "Not all medication is perfectly suited to every child, even if they have the same medical condition or complaint (as an adult).
"So for children to get quality and effective medication that's right for them, it's important that healthcare professionals work with families to report all side-effects, no matter how big or small."