Add tags to long page that is broken up into subpages?

You have to add tag support to your Pages.

function page_tags() {
   register_taxonomy_for_object_type('post_tags','pages'); // adds tag support
   add_meta_box('tagsdiv-page_tag','tags','post_tags_meta_box','page','side','low'); // adds the meta box itself
}
add_action('admin_init','page_tags');

You still won’t see tags anywhere. You can move the meta box around using the ‘side’ and ‘low’ parameters. See the link below.

Now look in TwentyEleven’s content-single.php and find the get_the_tag_list line. That is the part you want. You need to edit content-page.php to look like …

<footer class="entry-meta">
    <?php echo get_the_tag_list( '', __( ', ', 'twentyeleven' ) ); ?>
    <?php edit_post_link( __( 'Edit', 'twentyeleven' ), '<span class="edit-link">', '</span>' ); ?>
</footer><!-- .entry-meta --> 

That should do it.

Be aware that editing a default theme is dangerous. It will be overwritten when you update. You should be creating a child theme.

http://codex.wordpress.org/Function_Reference/register_taxonomy_for_object_type

http://codex.wordpress.org/Function_Reference/add_meta_box