Wordpress create tag list of all child pages.

<?php

		/*--------------------------------
		|*	Grab the sub pages
		|*------------------------------- */
		$sub_pages = new WP_Query(
			array( 
				'post_parent' => $post->ID,
				'post_type' => 'page'
			)
		);
		$sub_page_posts = $sub_pages->posts;

		/*--------------------------------
		|*	Sort out the tags.
		|*------------------------------- */
		
		$tag_ids = [
			[
				'slug' => 'all',
				'name' => 'All'
			]
		];
		foreach( $sub_page_posts as $sub_page):

			$sub_page->tags = get_the_tags( $sub_page->ID );

			/*--------------------------------
			|*	Create an empty var to hold 
			|*	all the slugs to add as a class
			|*------------------------------- */
			$sub_page->tag_classes = ['filterable'];

				if($sub_page->tags):
					foreach($sub_page->tags as $tag):
						
						/*--------------------------------
						|*	Add the tag to the var
						|*------------------------------- */

						$sub_page->tag_classes[] = $tag->slug;
						
						/*--------------------------------
						|*	And, if we don't have it, 
						|*	store what we need for the menu
						|*------------------------------- */

						if( ! array_key_exists($tag->term_id, $tag_ids)):
							$tag_ids[$tag->term_id] = [
								'slug' => $tag->slug,
								'name' => $tag->name
							] ;
						endif;

					endforeach;
				endif;
		endforeach;
			
		?>
		<div class="sidebar-module">
			<h6>Categories</h6>
			<nav class="no-bullets side-ruled-menu">
				<ul id="filters">
				<?php if($tag_ids): ?>
					<?php foreach($tag_ids as $tag): ?>
						<li><a href="<?php echo home_url('/') . '?tag=' . $tag['slug'] ;?>" data-tag="<?php echo $tag['slug']; ?>"><?php echo $tag['name']; ?></a></li>
					<?php endforeach; ?>
				<?php endif; ?>
				</ul>
			</nav>
		</div>