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
->addAttributeToSelect(array('name', 'image', 'description'))
Getting child categories with flat categories enabled
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