Set page template automatically based on parent

Question

As the title implies, I'm trying to set each new page under X parent to a specific page template. Either through a function/plugin/code

eg: if I make a 'swiss cheese' page, that is a children of the 'chesses' page, wp will automatically assign it the 'cheese' page template

Answer 1

On the admin side, you could update the meta data for the page-post_type programmatically:

global $post;
if ( 
    'swiss_cheese' === $post->post_parent 
    AND is_admin()
)
    update_post_meta( $post->ID, '_wp_page_template', 'some_template.php' );

On the visitor facing side, you can simply jump into template redirect:

function wpse63267_template_redirect()
{
    global $post;

    if ( 'swiss_cheese' === $post->post_parent )
    {
        include ( get_stylesheet_directory().'/swiss-cheese-template.php');
        exit;
    }
}
add_action( 'template_redirect', 'wpse63267_template_redirect' );