image

$_product = Mage::getModel('catalog/product')->load(108);//categoty id
 $conf = Mage::getModel('catalog/product_type_configurable')->setProduct($_product);
 $col = $conf->getUsedProductCollection()->addAttributeToSelect('*')->addFilterByRequiredOptions();
 foreach($col as $simple_product){
     var_dump($simple_product->getId());
 }