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class Demac_Childsku_Model_Observer
{
    public function addChildSkus()
    {
        $productConfig = Mage::getResourceModel('catalog/product_collection')->addAttributeToFilter('type_id', 'configurable');
        foreach ($productConfig as $parent){
             $singleConfig = Mage::getModel('catalog/product_type_configurable')->setProduct($parent);
             $simpleCollection = $singleConfig->getUsedProductCollection()->addAttributeToSelect('*');

             foreach($simpleCollection as $simple){
             //Do something here to modify your parent product.
             }
        }
    }
}