To create a category with some child categories, you need to perform the following steps:

1. Create a method in the library.book.category model with the name create_dummy_category:

def create_categories(self):
    ......

2. Inside the body of this method, prepare a dictionary of values for the fields of the first child category:

categ1 = {
    'name': 'Child category 1',
    'description': 'Description for child 1'
}

3. Prepare a dictionary of values for the fields of the second category:

categ2 = {
    'name': 'Child category 2',
    'description': 'Description for child 2'
}

4. Prepare a dictionary of values for the fields of the parent category:

parent_category_val = {
    'name': 'Parent category',
    'email': 'Description for parent category',
    'child_ids': [
        (0, 0, categ1),
        (0, 0, categ2),
    ]
}

5. Call the create() method to create the new records:

record = self.env['library.book.category'].create(parent_category_val) 

6. Add a button in the <form> view to trigger the create_categories method from the user interface:

<button name="create_categories" string="Create Categories" type="object"/>