How to use "taxonomy_get_childern" function
In Drupal 7 you can use this code


  $nid = arg(1);//get the arugument
  if (arg(0) == 'node'  && is_numeric($nid)) {//check for node page
    $query = db_select('field_data_field_books', 'book');
    $query->fields('data', array('tid'));
    $query->innerJoin('taxonomy_term_data', 'data', 'data.tid = book.field_books_tid');
    $query->condition('book.entity_id', $nid, '=');
    $tid = $query->execute()->fetchField();//only one field
    $child = taxonomy_get_children($tid);//get your children
    foreach ($child as $ch) {
      print "Name===>" . $ch->name;
    }
  }