All children may experience very stressful events that affect how they think and feel. Most of the time, children recover quickly and well. However, sometimes children who experience severe stress, such as from an injury, from the death or threatened death of a close family member or friend, or from violence, will be affected long-term. The child could experience this trauma directly or could witness it happening to someone else. When children develop long term symptoms (longer than one month) from such stress, which are upsetting or interfere with their relationships and activities, they may be diagnosed with post-traumatic stress disorder (PTSD).
Sourced from the CDC