If you enable the flat categories in your Magento store, some code that gets children of a given category might stop working. The following code can be used to get all the cildren of a given category with enabled flat categories:

$category = Mage::getModel('catalog/category')->load($id);    
$childCategories = $category
                    ->getCollection()
                    ->addAttributeToSelect(array('name', 'image', 'description'))
                    ->addIdFilter($category->getChildren());