Autocheck all checkboxes within dynamic html div

So basically i want to check all the check-boxes within a html div if the "parent checkbox" for that div is checked. I'm new at javascript/php/html. can anyone provide an useful explanatory example?

Here's my code:

    <form>
  <?php
    while(list($k, $v)=each($Aff))
    {
    if ($k == 0)
    {
      array_push($parentAff, substr($v, 0, 2));
      $substring = $v;
      echo ('<div id ="Div'.$v.'">');
    }
    if ((substr($substring, 0, 2) != substr($v, 0, 2)) && (strlen($substring) != 1))
    {
      echo ('</div>');
      echo ('<div id ="'.$v.'">');
      array_push($parentAff, substr($v, 0, 2));
      $counter++;
      $substring = $v;
      echo "<hr>";
    }

    echo ('<input type="checkbox" name="Aff[]" id="'.$v.'" value="'.$v.'" /><label for="text'.$k.'">'.$v.'</label>');


    $substring = $v;

    }

    echo ('</div>');

  ?>
</form>

The number of checkboxes within a div depend on what data comes out of the database to the array Aff[]. the parent checkbox for each div would be the one in the parentAff array wich is identified by the div id.