[Plugin Folding Pages] Help Customizing PHP

I’m using ‘Folding Pages’ plugin for my sidebar navigation. Works well, but I’d like to customize this plugin’s PHP (see below) to ONLY list the child pages of my services page (ID=13) on every single page. Once a child page is clicked, it should expand to the child-child pages as it does now.

View example of site in progress. Obviously unfinished. https://inktrigue.com/wp/?page_id=13

I feel like it’s not too complicated, but I’m super new w/PHP. Any ideas? Thanks…!!

<?php
/*
Plugin Name: Folding Pages Widget
Plugin URI: https://navyroad.com/wordpress-plugins/nrs-folding-pages-widget/
Description: Provides a hierarchical list of pages for your sidebar.
Author: Chris Carson
Version: 1.0
Author URI: https://navyroad.com
Credits: Thanks to Andy Skelton.  Based on code by Erwin Harte (https://is-here.com/projects/wordpress/pages) Calvin Yu (https://blog.codeeg.com/)
*/

function nrs_folding_pages($args){
    extract($args);
    //var_dump($args);
    $sort_opts = nrs_folding_pages_get_sort_opts();
    $options = get_option("nrs_folding_pages");
    $title = (empty($options["title"])) ? null : $options["title"];
	$sort_column = (array_key_exists($options["sort_column"],$sort_opts)) ? $options["sort_column"] : key($sort_opts);

    global $notfound;
	global $post;

	//returns a numerically indexed array...
	$pages = get_pages("sort_column=$sort_column");

	//a new associative array indexed by ID...
	$nodes = array();
	foreach ($pages as $page) $nodes[$page->ID] = $page;

	//an array to hold the parent ids in the current tree...
	$parent_ids = array();
	if (is_page() and ($notfound != '1'))
	{
		$curr_page_id = $post->ID;
		while ($curr_page_id)
		{
			array_unshift($parent_ids, $curr_page_id );
			$curr_page_id = $nodes[$curr_page_id]->post_parent;
		}
	}
	//add the top-level parent id, which is 0...
	array_unshift($parent_ids, 0 );

	//output...
	echo $before_widget;
	echo  $before_title . $title . $after_title;
	nrs_display_tree($parent_ids, $nodes, 0);
	echo $after_widget;
}
function nrs_display_tree($parent_ids, $nodes, $curr_id)
{
	global $post;
	if ($curr_id)
	{
		$node = $nodes[$curr_id];
		$title = wp_specialchars($node->post_title);
		$link = get_page_link($curr_id);
		$class = "page_item";
		if ($curr_id == $post->ID) $class .= " current_page_item";
		echo "<li class=\"$class\"><a href=\"$link\" title=\"$title\">$node->post_title</a>";
	}
	if (in_array($curr_id, $parent_ids))
	{
		$num_children = 0;
		foreach ($nodes as $node)
		{
			if ($node->post_parent == $curr_id)
			{
				if (! $num_children) echo "\n<ul>";
				nrs_display_tree($parent_ids, $nodes, $node->ID);
				$num_children++;
			}
		}
		if ($num_children) echo "</ul>\n";

	}
	if ($curr_id) echo "</li>\n";

}

function nrs_folding_pages_get_sort_opts() {
	return array( "post_title"=>"Title",
					"menu_order"=>"Page Order");
}
function nrs_folding_pages_control() {
	$sort_opts = nrs_folding_pages_get_sort_opts();
    $options = get_option("nrs_folding_pages");

    if ( $_POST['nrs_folding_pages_submit'] )
    {
		$options['title'] = strip_tags(stripslashes($_POST['nrs_folding_pages_title']));
		$options['sort_column'] = (array_key_exists($_POST['nrs_folding_pages_sort_column'], $sort_opts)) ? $_POST['nrs_folding_pages_sort_column'] : key($sort_opts);
		update_option('nrs_folding_pages', $options);
    }
    $sort_column = (array_key_exists($options["sort_column"],$sort_opts)) ? $options["sort_column"] : key($sort_opts);
    $title = wp_specialchars($options['title']);

?>
	<input type="hidden" id="nrs_folding_pages_submit" name="nrs_folding_pages_submit" value="1" />
	<p><label for="nrs_folding_pages_title"><?php _e('Title:'); ?> <input style="width: 250px;" id="nrs_folding_pages_title" name="nrs_folding_pages_title" type="text" value="<?php echo $title; ?>" /></label></p>
	<p><label for="nrs_folding_pages_sort_column">
		<?php _e('Sort:'); ?>
		<select  id="nrs_folding_pages_sort_column" name="nrs_folding_pages_sort_column">
		<?php
		foreach ($sort_opts as $val=>$label){
			echo "<option value=\"$val\"";
			if ($sort_column == $val) echo " selected";
			echo ">$label</option>";
		}
		?>
		</select>
	</label></p>

<?php
}
function nrs_folding_pages_init()
{

    register_sidebar_widget("NRS Folding Pages", "nrs_folding_pages");
    register_widget_control("NRS Folding Pages", "nrs_folding_pages_control");
}

add_action("plugins_loaded", "nrs_folding_pages_init");

?>