Essential checks that alert emergency doctors to a deterioration in a child's health are not always recorded, a new report suggests.
Youngsters who go to A&E with medical illnesses, as opposed to an injury, are supposed to have their vital signs - such as temperature, breathing and heart rate - recorded in their notes within 15 minutes.
Those who have abnormal vital signs are supposed to have another full assessment within an hour.
A new audit from the Royal College of Emergency Medicine (RCEM) concluded "there is a need for increased documentation of both initial and repeat vital signs" within the appropriate timeframes.
The college warned many young children are unable to communicate their symptoms, so recording vital checks is an important aspect of care.
The audit also found 30% of emergency departments across the UK are not using formalised scoring systems which can alert medics when a child takes a turn for the worse.
The college, which examined data for 16,000 children under 16 attending 191 emergency departments across the UK, recommends medics should use the Paediatric Early Warning Scores (PEWS) system, or an equivalent early warning score, to record results.
RCEM president Dr Cliff Mann said: "This audit confirms that there is much good practice in emergency departments but highlights disparate assessment methods for these patients.
"The audit shows that one-third of children presenting to emergency departments are infants - those below two-years-old who have limited ability to communicate symptoms and are therefore the most challenging.
"Paediatric emergency medicine is particularly challenging because we know there will be a few very sick children amongst the many children with similar symptoms who have a self-limiting illness - the needles in the haystack.
"In the paediatric population we know that standardised assessment and scoring methods can help clinicians spot the sick children but no tool is currently sufficiently sensitive or specific.
"To enable sick children to be identified, there is a clear need to agree a standardised scoring method that all clinicians can use."
Dr John Criddle, chair of the Royal College of Paediatrics and Child Health's emergency care committee, added: "Children make up nearly a quarter of total attendances at A&E and a significant proportion present with conditions such as fever, wheeze and breathing difficulties.
"Often, these conditions are dealt with quickly and easily but sometimes there can be underlying issues which can go unrecognised if vital signs are not recorded - sometimes with grave consequences.
"We, therefore, welcome the recommendations made by the RCEM in today's report which reiterate the Intercollegiate Standards for Children and Young People in Emergency Care Settings.
"All children and young people should receive an initial assessment within 15 minutes of arrival to ensure that we minimise the likelihood of missing serious illness in this challenging group of patients."