A Parent Child relationship is a relationship between two URLs, a legacy version and a new version. When setup, the legacy URL attributes data to the new URL and the legacy version is no longer displayed in data tables. Data is attributed to the new URL from the time the relationship was setup.  The new parent URL will take over any keyword associations from the legacy child URL. Page tags are not updated automatically. 

Parent Child relationships can be setup via the Technical tab of Page Clarity Detail.