This is how you can get all simple products linked from a configurable product
$configurableProduct = Mage::getModel('catalog/product')->load(1);
$childProducts = Mage::getModel('catalog/product_type_configurable')->getUsedProducts(null,$configurableProduct);
foreach($childProducts as $child) {
echo $child->getId();
}
If you only need ids of the child product, you can do this
$childProductsId = Mage::getModel('catalog/product_type_configurable')->getChildrenIds(1);