How to create python C++ extension with submodule that can be imported

I'm creating a C++ extension for python. It creates a module parent that contains a sub-module child. The child has one method hello(). It works fine if I call it as

import parent
parent.child.hello()
> 'Hi, World!'

If I try to import my function it fails

import parent
from parent.child import hello
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> ModuleNotFoundError: No module named 'parent.child'; 'parent' is not a package

parent.child
> <module 'child'>

here is my code setup.py

from setuptools import Extension, setup
  
# Define the extension module
extension_mod = Extension('parent',
                          sources=['custom.cc'])

# Define the setup parameters
setup(name='parent',
      version='1.0',
      description='A C++ extension module for Python.',
      ext_modules=[extension_mod],
      )

and my custom.cc

#include <Python.h>
#include <string>

std::string hello() {
    return "Hi, World!";
}

static PyObject* hello_world(PyObject* self, PyObject* args) {
    return PyUnicode_FromString(hello().c_str());
}

static PyMethodDef ParentMethods[] = {
    {nullptr, nullptr, 0, nullptr}
};

static PyMethodDef ChildMethods[] = {
    {"hello", hello_world, METH_NOARGS, ""},
    {nullptr, nullptr, 0, nullptr}
};

static PyModuleDef ChildModule = {
    PyModuleDef_HEAD_INIT,
    "child",
    "A submodule of the parent module.",
    -1,
    ChildMethods,
    nullptr,
    nullptr,
    nullptr,
    nullptr

};

static PyModuleDef ParentModule = {
    PyModuleDef_HEAD_INIT,
    "parent",
    "A C++ extension module for Python.",
    -1,
    ParentMethods,
    nullptr,
    nullptr,
    nullptr,
    nullptr
};

PyMODINIT_FUNC PyInit_parent(void) {
    PyObject* parent_module = PyModule_Create(&ParentModule);
    if (!parent_module) {
        return nullptr;
    }
    PyObject* child_module = PyModule_Create(&ChildModule);
    if (!child_module) {
        Py_DECREF(parent_module);
        return nullptr;
    }

    PyModule_AddObject(parent_module, "child", child_module);

    return parent_module;
}

I install build and install with python setup.py build install.

So, how do I make sure that my parent is a package?

from How to create python C++ extension with submodule that can be imported