$ids = array($_GET['id']); // or whatever

while ($id = array_pop($ids)) {
  $qry = mysql_query('SELECT id FROM module_comments WHERE parent_id = '.$id);
  while ($new_id = mysql_fetch_assoc($qry)) $ids[] = array_pop($new_id);
  mysql_query('DELETE * FROM module_comments WHERE id = '.$id);
}