Add new page with preselected parent

welcome to WPSE!

This answer from Mike Schinkel was useful to find a solution^* for Get QueryString values with jQuery. *^{(not used anymore, a simple $_GET does it)}

It’s also worth noting that, in that same answer, Mike also links to this other important Question: What is the best way to add custom javascript files to the site?

That being said, the following code does the trick. It doesn’t use admin_enqueue_scripts -best practice-, and instead is printing it directly in the admin footer…

Add this code to your theme’s functions.php and make your custom link to “Add New Page” like this: http://example.com/wp-admin/post-new.php?post_type=page&the_id=28, being the_id the ID of the desired parent page.

add_action( 'admin_head-post-new.php', 'pre_select_parent_wpse_56952' );

function pre_select_parent_wpse_56952() 
{
    // Check if adding new page 
    if( !isset($_GET['post_type']) || 'page' != $_GET['post_type'] ) 
        return;

    // Check for pre-selected parent
    if( !isset($_GET['the_id']) || empty( $_GET['the_id'] ) ) 
        return;

    // There is a pre-selected value for the correct post_type, proceed with script
    $the_id = $_GET['the_id'];
    ?>
        <script type="text/javascript">
        jQuery(document).ready( function($) 
        {
            $('#parent_id').val(<?php echo $the_id; ?>);
        });
        </script>
    <?php
}