data = [
    {
        'id': '1',
        'parent': '',
        'children': [
            {
                'id': '2',
                'parent': '1',
                'children': [
                    {
                        'id': '8',
                        'parent': '2',
                        'children': [
                            {
                                'id': '9',
                                'parent': '8'
                            }
                        ]
                    }
                ]
            },
            {
                'id': '3',
                'parent': '1'
            }
        ]
    },
    {
        'id': '4',
        'parent': '',
        'children': [
            {
                'id': '5',
                'parent': '4'
            },
            {
                'id': '6',
                'parent': '4'
            },
            {
                'id': '7',
                'parent': '4'
            }
        ]
    }
]


def nested2flat(nested: list) -> list:
    res = []
    for child in nested:
        children = child.pop('children', [])
        res.append(child)
        res.extend(nested2flat(children))
    return res


if __name__ == '__main__':
    print(nested2flat(data))

"""
result = [
    {'id': '1', 'parent': ''},
    {'id': '2', 'parent': '1'},
    {'id': '8', 'parent': '2'},
    {'id': '9', 'parent': '8'},
    {'id': '3', 'parent': '1'},
    {'id': '4', 'parent': ''},
    {'id': '5', 'parent': '4'},
    {'id': '6', 'parent': '4'},
    {'id': '7', 'parent': '4'}
]
"""