If you are using a child theme then maybe you should try the following code. Not necessary to put !important
too unless some other plugin or code is overwriting it.
function custom_loginlogo() {
echo '<style type="text/css">
h1 a { background-image: url('. get_stylesheet_directory_uri().'/images/login_logo.png); }
</style>';
}
add_action('login_head', 'custom_loginlogo');