Select * FROM SYSADM.PSPRSMDEFN where PORTAL_PRNTOBJNAME = ‘MY_PARENT_OBJECT’;
This gives all the objects directly below 'MY_PARENT_OBJECT'. The object type is stipulated by a C' for component or 'F' for folders; there are another couple of types for links etc... In order to find all the objects below another you would need a resursive function that drills down each level because for each object, you only have its parent.