Load Simple products by configurable parent id

sandbox.php

<?php

//Where $parent is your configurable product object
$configurableProduct = Mage::getModel('catalog/product_type_configurable')->setProduct($parent);
$simpleCollection = $configurableProduct->getUsedProductCollection()->addAttributeToSelect('*');

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