Single Page Navigation of grandchildren skipping parents

gistfile1.php

<?php
function lm_do_video_navigation() {

	// This runs on the grandchild.  It finds the grandparent, then it's children, and loops through
	// the children in order building an array of the grandchildren in menu_order.

	// get top parent post ID
	$top_parent_ID = get_page(array_pop(get_post_ancestors($post->ID)))->ID;
	
	$args = array( 
		'post_type' 	=> 'course',
        'child_of' => $top_parent_ID, 
        'parent' => $top_parent_ID,
        'hierarchical' => 0,
        'sort_column' => 'menu_order', 
        'sort_order' => 'asc'
	);
	$sections = get_pages( $args );
	
	// Initialize array used inside nested 
	$video_IDs = array();
	
	// We need to build the video id array by sections because menu_order may not span sections
	// otherwise we could use post_parent__in array and use one query :(
	foreach ($sections as $section) { 

		$args = array(
			'post_type' 	=> 'course',
			'parent' => $section->ID, 
			'number' => 50, // Return all, but limit for sanity check
			'sort_column' 	=> 'menu_order',
			'sort_order'	=> 'ASC',
			'fields'		=> 'ids',
		);
		
		$videos = get_pages($args);
		
		// Turn post objects into an array of IDs
		foreach ($videos as $video) {
			array_push($video_IDs, $video->ID);
		}
	}

	$current = array_search(get_the_ID(), $video_IDs);

	$prevID = $video_IDs[$current-1];
	$nextID = $video_IDs[$current+1];

	echo '<div class="pagination">';
	if (!empty($prevID)) {
		echo '<div class="alignleft">';
		echo'<a href="' . get_permalink($prevID) . '" title="' . get_the_title($prevID) . '">Previous Video<br/><span class="pagination-title">' . get_the_title($prevID) . '</span></a>';
		echo '</div>';
	}
	if (!empty($nextID)) {
		echo '<div class="alignright">';
		echo'<a href="' . get_permalink($nextID) . '" title="' . get_the_title($nextID) . '">Next Video<br/><span class="pagination-title">' . get_the_title($nextID) . '</span></a>';
		echo '</div>';
	}
	echo '</div><!-- .pagination -->';
}