WordPress Custom Post Type Children Template

If, from within single-films.php you filter based on the parent post you should have little issue with this. For example:

if( $post->post_parent != 0 ) {
    // not top level
} else {
    // top level page
}

If you need to know how deep the page is, that’s a different matter, but that’s also doable (unfortunately it means you need to query the database though).