Create WordPress child page from page actions

If you mean the 404 error:

function slt_childPageAction( $actions, $page ) {
    $actions["create-child"] = '<a href="'.add_query_arg(array('post_type' => 'page', 'parent_id' => $page->ID), admin_url('post-new.php')).'" title="Create a new page with this page as its parent">Create child</a>';
    return $actions;
}
add_filter( 'page_row_actions', 'slt_childPageAction', 10, 2 );
function slt_setChildPage() {
    global $post;
    if ( $post->post_type === 'page' && $post->post_parent === 0 && isset( $_GET["parent_id"] ) )
        echo '<script type="text/javascript">jQuery( document ).ready( function($) { $("#parent_id").val("' . (int)$_GET["parent_id"] . '"); } );</script>';
}
add_action( 'edit_page_form', 'slt_setChildPage' );

(the url path to /wp-admin/ was hard-coded)

nice find btw 🙂