Actually, the problem is that you're iterating through task_structs when you should be iterating through struct list_heads:

  struct list_head* head = &this->children;
  struct list_head* i = head->next;
  for(; i!=head ;i=i->next)
      {
      task_struct* child = container_of(i,task_struct,sibling);
      /* do stuff */
      }