Magento 1.9 Category Issue – $category->getChildren() Not Working

if you are in the category view page you can do the following.

$mainCategory = Mage::registry('current_category');
foreach ($mainCategory->getChildrenCategories() as $category){
    $collection = $category->getProductCollection()->addAttributeToSelect('image');//or thumbnail or small_image
    Mage::getSingleton('catalog/product_status')->addVisibleFilterToCollection($collection);
    Mage::getSingleton('catalog/product_visibility')->addVisibleInCatalogFilterToCollection($collection); 
    $collection->addAttributeToSort('position');//or 'name' or any other attribute
    $product = $collection->setPage(1, 1)->getFirstItem();
    if ($product->getId()){//if the category has at least a valid product
       $imageUrl = Mage::>helper('catalog/image')->init($product, 'image')->resize(60);
       //do something with $imageUrl
    }
}

there is a trick involved as Magento only allows category save on EAV structure and it differences this (if you ask for category model) in code by checking admin store scope so you have to fake the scope to be able to save the categories if you are doing this outside of admin

$currentStore = Mage::app()->getStore()->getId();
Mage::app()->getStore()->setId(Mage_Core_Model_App::ADMIN_STORE_ID);
$parent = Mage::getModel("catalog/category")->load(50);
$parent->setCustomUseParentSettings(false);
$parent->setCustomApplyToProducts(false);
$parent->setPageLayout("two_columns_left");
$parent->save();
// set the store scope back
Mage::app()->getStore()->setId($currentStore);