How to add programmatically all pages and children to backend menu

The helpers you’ll need are wp_create_nav_menu() and wp_update_nav_menu_item().

This will allow you to create a menu and then, using a query posts loop, programmatically add every page in your site dynamically.

You can use the get_pages() helper to retrieve all pages from the DB.

<?php
// These are default args and don't need to be set explicitly.
// I've included them here for reference.
$args = array(
    'hierarchical' => 1,
    'post_type' => 'page',
    'post_status' => 'publish'
);

$pages = get_pages($args);

// loop goes here
// foreach or while will work, whatever you are comfortable with
?>