If the parent (page type 1) and child pages (page type 2) are sharing the same template, and the field value exist on the child page type, then if you have the {% FieldName %} in a web part's HTML envelope (for example), then it should display nothing on parent, but display something on child page. You don't need to do anything special.
Also, I just noticed for the visible condition macro, you don't need {% if (showOptionalContent == true) { return true; } else { return false; } |(identity)GlobalAdministrator%}
because it would be true or false or 0 or 1.