How to add a class to a checkbox in a form?
Sure.  You can copy theme_checkbox() from includes/form.inc:

function theme_checkbox($variables) {
  $element = $variables['element'];
  $element['#attributes']['type'] = 'checkbox';
  element_set_attributes($element, array('id', 'name', '#return_value' =&gt; 'value'));

  // Unchecked checkbox has #value of integer 0.
  if (!empty($element['#checked'])) {
    $element['#attributes']['checked'] = 'checked';
  }
  _form_set_class($element, array('form-checkbox'));

  return '&lt;input' . drupal_attributes($element['#attributes']) . ' /&gt;';
}


into your theme's template.php file and rename it accordingly:

function YOURTHEME_checkbox($variables) {
  $element = $variables['element'];
  $element['#attributes']['type'] = 'checkbox';
  element_set_attributes($element, array('id', 'name', '#return_value' =&gt; 'value'));

  // Unchecked checkbox has #value of integer 0.
  if (!empty($element['#checked'])) {
    $element['#attributes']['checked'] = 'checked';
  }
  _form_set_class($element, array('form-checkbox', 'checkradios')); // &lt;-- checkradios added in here

  return '&lt;input' . drupal_attributes($element['#attributes']) . ' /&gt;';
}


along with the addition of the checkradios class and this will be added to each checkbox.

Now, if you want to restrict it to certain fields, you can look inside $element and add some additional logic to this.