Magento Category Page – How to Add Two Static Blocks

In your code to get the children categories from current category, which looks like this :

<?php
    $layer = Mage::getSingleton('catalog/layer');
    $_category   = $layer->getCurrentCategory();
    $_categories = $_category->getCollection()->addAttributeToSelect(array('url_key','name','image','all_children','is_anchor','description'))
        ->addAttributeToFilter('is_active', 1)
        ->addIdFilter($_category->getChildren())
        ->setOrder('position', 'ASC')
        ->joinUrlRewrite();
?>

Try with this code instead :

<?php
    $layer = Mage::getSingleton('catalog/layer');
    $_category   = $layer->getCurrentCategory();
    $_categories = $_category->getCollection()->addAttributeToSelect(array('url_key','name','image','all_children','is_anchor','description'))
        ->addAttributeToFilter('is_active', 1)
        ->addIdFilter($_category->getAllChildren(true))
        ->setOrder('position', 'ASC')
        ->joinUrlRewrite();
?>

Difference is, your call to method getChildren returns a comma separated list of IDs, not an array, while the method addIdFilter expects an array to be passed. When you pass true to the method getAllChildren, it will return the list as an array.

I found the answer here, in another question. To clarify further; I went with the first approach, created a simple module and used the code from the said answer in my view/frontend/templates/subcategories.phtml Then I use a custom layout update XML in the adminpanel for the categories where I want the code to show.