Showing a block in specific content types and URLs
If you want to check all children of specified term below code can help you

first load all children of it ,then check its term ids one by one

$tids=array_keys(taxonomy_get_children(8));
foreach($tids as $tid)
 if (strpos($url, "taxonomy/term/".$tid)) {
    $match = TRUE;
    break;
  }