Children may avoid stressful situations
Children as young as two experience post-traumatic stress, research shows.
A study on 114 younger children who had been exposed to road traffic accidents in the UK found one in 10 suffered continued anxiety after the event.
Although this is similar to the rate seen in adults, most go unrecognised and untreated, say the King's College London experts.
Their work is published in the latest edition of the American Journal of Psychiatry.
One reason for the lack of knowledge about young children is the difficulty in making psychiatric diagnoses in this age group, as they frequently lack the language ability to talk about their feelings and experiences.
And the tools used by doctors to spot and measure post-traumatic stress disorder (PTSD) were designed with adults in mind.
The researchers, from King's and the Medical Research Council's Cognition and Brain Science Unit Cambridge, used an age-appropriate technique for diagnosing PTSD in the young children that relied on parents' reporting of how their offspring were coping.
All of the 114 children aged between two and 10 had visited A&E departments in London after a road traffic accident.
Half had been passengers in a car, others were pedestrians or cyclists struck by a car, and all had relatively minor physical injuries.
At assessment in the month following the accident, and again six months later, more than 10% of the children met conditions for a diagnosis of PTSD.
These children had nightmares and difficulty sleeping, displayed avoidance behaviours, such as not wanting to go out in the car or walk on busy roads, and were described by their parents as "jumpy" and "on edge".
Flashbacks and nightmares
Avoidance from stressful situations
Increased arousal and feelings of being under constant threat
The symptoms are persistent and disturb everyday life
Lead researcher Dr Richard Meiser-Stedman, from King's Institute of Psychiatry, said: "Our findings indicate that the mental health needs of pre-school children caught up in terrifying events should be considered by parents and health services.
"This is especially important as young children are not able to access health services on their own and are at such a vulnerable point in their development.
"More research is needed into how PTSD in young children should be treated and how parents can best help their offspring in the aftermath of a trauma."
Professor David Cottrell, of Young Minds and a child and adolescent psychiatrist at Leeds University, said: "Sadly, children experience a lot of traumatic events. However, it is important to remember that the vast majority of children will not develop symptoms and will not need any intervention after a stressful event.
"Children are quite resilient and often parents and close relatives are the best therapy."