Webform theming field labels through webform-form.tpl.php
You can simply hide the label from date field setting and add like this : 

&lt;div class="date-wrapper"&gt;
  &lt;label&gt;Label here&lt;/label&gt;
  &lt;div class="date-field"&gt;
    &lt;?php print drupal_render($form['submitted']['birthdate']['day']); ?&gt;
    &lt;?php print drupal_render($form['submitted']['birthdate']['month']); ?&gt;
    &lt;?php print drupal_render($form['submitted']['birthdate']['year']); ?&gt;
  &lt;/div&gt;
&lt;/div&gt;