code: def _get_id2children(id2children, item_id, item_obj):
      
      if item_id in id2children:
          return id2children[item_id]
      child_ids = set()
      for child_obj in item_obj.children:
          child_id = child_obj.item_id
          child_ids.add(child_id)
          child_ids |= _get_id2children(id2children, child_id, child_obj)
      id2children[item_id] = child_ids
      return child_ids
docs: Add the child item IDs for one item object and their children.