<?php
// input is $_product and result is iterating child products
$conf = Mage::getModel('catalog/product_type_configurable')
       ->setProduct($_product);
$col = $conf->getUsedProductCollection()
->addAttributeToSelect('*')->addFilterByRequiredOptions();
foreach($col as $simple_product){
    var_dump($simple_product->getId());
}
?>