Even full grown adults have this under immense stress/shock. Emergency response medics, and others who work in emergency conditions, such as police, ER staff, firefighters, etc., are far more conditioned to viewing horrific scenes, and are far less likely to relieve themselves in such a way. But it is common for people who have been in car accidents, witnessed a horror, or something else that has given them a serious shock, to relieve themselves involuntarily.
they're still very young, so they haven't yet gained full control of their blatter abilities. and it's not always children. sometimes, when adults are scared enough, they could wet themselves or even soil their pants.