您的问题的解决方案如下.我不会使用mysql rand()函数，因为它相当慢.

The solution to your question is bellow. I wouldn't use mysql rand() function since it's quite slow.

<ul class="brand_list">
  <?php 
    $media = Mage::getBaseUrl(Mage_Core_Model_Store::URL_TYPE_MEDIA);
    $parentCategory = Mage::getModel('catalog/category')->load(12);
    $children = Mage::getModel('catalog/category')
                  ->getCollection()
                  ->addIdFilter( array_rand( array_flip( $parentCategory->getAllChildren( true ) ), 6) )
                  ->addAttributeToSelect('name')
                  ->addAttributeToSelect('thumbnail');
  ?>
  <?php foreach ($children as $category): ?>
    <li class="span3">
      <a href="<?php echo $category->getUrl(); ?>">
        <img alt="<?php echo $category->getName(); ?>" src="<?php echo $media; ?>catalog/category/<?php echo $category->getThumbnail(); ?>" />
      </a>
    </li>
  <?php endforeach; ?>
</ul>