Add wagtail child page in migrations

treebeard_migration_utils.py

def add_child_page(parent_page, instance):
    """
    Mimic the behavior of the "add_child" from Django treebeard.

    We can't use `parent_page.add_child(instance=child_page)` inside of
    migrations because historical models don't have access to class
    methods. This replicates the behavior of `add_child` by calculating
    the treebeard path, path depth and url path.

    https://django-treebeard.readthedocs.io/en/latest/api.html#treebeard.models.Node.add_child
    """
    child_page = instance
    child_page_class = child_page.__class__

    treebeard_path = '%s00%02d' % (parent_page.path, parent_page.numchild + 1)
    url_path = '%s%s/' % (parent_page.url_path, child_page.slug)

    child_page.path = treebeard_path
    child_page.depth = parent_page.depth + 1
    child_page.numchild = 0
    child_page.url_path = url_path
    child_page.save()

    parent_page.numchild += 1
    parent_page.save()

    return child_page