Questions about hierarchical taxonomies

Hello, I couldn’t search out the information anywhere else, need somebody shade lights…
1. Should I register both parent and child as “hierarchical => true”, if there’s no grandchild?
2. How to register child taxonomies?
If “Movie” is the top level taxonomy, when I register a child,
Is it as simply as :'parent_item' => __( 'Parent Movie' ) ?
3. How to automatically asign a child taxonomy to a post?
So far, I figured out how to asign a taxonomy to a post, but, does this work fine with child taxonomy?
wp_set_object_terms($post_ID, $cat, 'category');

Thanks!