code: def get_children(self, root=None, target=None):
          
          if root is None:
              print("Warning: TreeCtrl root/child must be given.")
              return None
          if target is None:
              print("Warning: TreeCtrl target must be given.")
              return None
          child, cookie = target.GetFirstChild(root)
          child_list = []
          while child.IsOk():
              if target.ItemHasChildren(child):
                  child_list.extend(self.get_children(child, target))
              child_list.append(child)
              child, cookie = target.GetNextChild(root, cookie)
          return child_list
docs: return list of all the children of root of given tree
              :param root: treectrl item, treeitemid
              :param target: treectrl object