code: def _get_all_children(self,):
          
          res = ''
          if self.child_nodes:
              for c in self.child_nodes:
                  res += ' child = ' + str(c) + '\n'
                  if c.child_nodes:
                      for grandchild in c.child_nodes:
                          res += '   child = ' + str(grandchild) + '\n'
          else:
              res += ' child = None\n'
          return res
docs: return the list of children of a node