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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 add_child_sensor(self, child_id, child_type, description=''):
 """Create and add a child sensor."""

        if child_id in self.children:
            _LOGGER.warning(
                'child_id %s already exists in children of node %s, '
                'cannot add child', child_id, self.sensor_id)
            return None
        self.children[child_id] = ChildSensor(
            child_id, child_type, description)
        return child_id