Updates the parent CPT's (in a hierarchical CPT mode) last modified time to that of it's last modified child.

functions.php

<?php
  add_action( 'save_post','changeLastModified' );
  function changeLastModified( $post ){
    if ( 'YOUR_CUSTOM_POST_TYPES_NAME' == get_post_type( $post ) ) {
      if( ! ( wp_is_post_revision( $post ) ) ) {
        $sync_time = array();
          $sync_time['ID'] = wp_get_post_parent_id( $post );
          $sync_time['post_modified'] = get_post_field( 'post_modified', $post, 'raw' );
        remove_action( 'save_post', 'changeLastModified' );
        wp_update_post( $sync_time, true );
        add_action('save_post', 'changeLastModified' );
      }
    }
  }
?>