Thank you very much for the feedback! Now I think, I found the solution. My final code is:
$parent_category_id = $this->getData('parent_category_id'); // 129
$number_of_columns = $this->getData('number_of_columns'); // 3
/** @var $_category Mage_Catalog_Model_Resource_Category_Collection */
$_categories = Mage::getModel('catalog/resource_category_collection')
->addAttributeToSort('name', 'ASC')
->addAttributeToSelect(array('name', 'url'))
->addIsActiveFilter()
->addFieldToFilter('parent_id', $parent_category_id);
$_page_size = (int) round($_categories->getSize() / $number_of_columns);
$_categories1 = clone $_categories;
$_categories1 = $_categories1->setPageSize(10)->setCurPage(1);
$_categories2 = clone $_categories;
$_categories2 = $_categories2->setPageSize(10)->setCurPage(2);
$_categories3 = clone $_categories;
$_categories3 = $_categories3->setPageSize(10)->setCurPage(3);
And then I can iterate through them separately:
<div class="column even" style="width: 25% !important;">
<div class="itemMenu level1">
<div class="title"><strong>Alphabetisch:</strong></div>
<?php foreach ($_categories1 as $_child): ?>
<a class="itemMenuName level1" href="<?php echo $_child->getUrl() ?>">
<span><?php echo $_child->getName() ?></span>
</a>
<?php endforeach; ?>
</div>
</div>
<div class="column odd" style="width: 25% !important; margin-top:2.75em;">
<div class="itemMenu level1">
<?php foreach ($_categories2 as $_child): ?>
<a class="itemMenuName level1" href="<?php echo $_child->getUrl() ?>">
<span><?php echo $_child->getName() ?></span>
</a>
<?php endforeach; ?>
</div>
</div>
<div class="column even" style="width: 25% !important; margin-top:2.75em;">
<div class="itemMenu level1">
<?php foreach ($_categories3 as $_child): ?>
<a class="itemMenuName level1" href="<?php echo $_child->getUrl() ?>">
<span><?php echo $_child->getName() ?></span>
</a>
<?php endforeach; ?>
</div>
</div>
<div class="clearBoth"></div>
I'm aware of the increase of memory consumption through clone, but hope, that there will be no other negative performance impacts. If you see any drawbacks for the solution, please, don't hesitate to post / ask.