Traverse from source GO down children.

def _traverse_child_objs(self, goobj_parent):
        """Traverse from source GO down children."""
        parent_id = goobj_parent.id
        # mark parent as seen
        self.seen_pids.add(parent_id)
        self.godag.go2obj[parent_id] = goobj_parent
        # Loop through children
        for child_obj in goobj_parent.children:
            child_id = child_obj.id
            self.godag.p2cs[parent_id].add(child_id)
            # If child has not been seen
            if child_id not in self.seen_pids:
                self._traverse_child_objs(child_obj)