//load parent product "$product"
$childProducts = Mage::getModel('catalog/product_type_configurable')
                ->getUsedProducts(null,$product);

// now get qty from collection
$qtyModel = Mage::getModel('cataloginventory/stock_item');
foreach($childProducts as $key => $child){
   $childQty[$key] = $qtyModel->loadByProduct($child)->getQty();
}