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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _get_all_children(self,):
 """ return the list of children of a node """

        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