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<?php
/**
 * Template Name: Redirect to child page
 * Description: Redirects to the top child page
 *
 * @package PUT YOUR THEME NAME HERE :D
 */

global $post;

$parent = $post->ID;

// In case you use WPML
if ( defined( 'ICL_LANGUAGE_CODE' ) ) {
    $_type  = get_post_type( $post->ID );
    $parent = icl_object_id( $post->ID, $_type, true, ICL_LANGUAGE_CODE );
}
$post_children = get_children( array(
    'posts_per_page' => 1,
    'orderby' => 'menu_order',
    'order' => 'ASC',
    'post_type' => 'page',
    'post_status' => 'publish',
    'post_parent' => $parent
) );
wp_redirect( get_permalink( array_pop( $post_children )->ID ), 301 );
exit;