If you are feeling distressed or anxious about the Martin Place incident help is available, including advice for supporting children, via the NSW Health website.
If you experience persistent issues that interfere with your life you are encouraged to talk to your General Practitioner or health care provider.
In NSW you can contact your local mental health service via the NSW Mental Health Line on 1800 011 511.
Lifeline is another great place for support on 13 11 14.
Traumatic events can affect children, adolescents and adults similarly, however, children may express their distress differently to adults, leading to misunderstanding.
The Australian Centre for Post Traumatic Mental Health has published guidelines for practitioners assisting children, adolescents and also adults who experience PTSD.
Check out the guidelines and other resources here: http://guidelines.acpmh.unimelb.edu.au/home