Traumatic incidents can of course involve children as well as adults and PTSD affects children too. Some people are surprised to learn this, but actually it is not surprising that following an overwhelming incident, accident or disaster children will also experience the symptoms of Post-traumatic Stress.
Particular traumatic occurrences such as physical or childhood abuse, the death of parents or the trauma of war are well known to have a profound effect on children. However, just as with adults, children can also be affected by a wide range of other traumatic events.
The reason why one does not often hear of PTSD in children is probably because children manifest symptoms of Post-traumatic Stress differently than do adults.