Previos Page | Next Page Navigation (not posts) within Children

Hi, I know this topic has been discussed several times. I checked forum posts before I come and I ask here. I am using WordPress 2.7 as a blog and CMS. I have a set of pages with their children organized in a hierarchy like this

Disease Categories
-Disease1
–Disease 1A
–Disease 1B

-Disease2
–Disease 2A
–Disease 2B

and so forth. You’ve probably figured it out already. I am designing a pathology page for my students. I want to have a simple previous page, next page navigation at the end of each page that let the student (user) move to the next page as if he/she was reading a book. I am not an expert in php scripting but I understand what loop is and how to edit template files so I can probably do it if someone helped me out.
So far this has been impossible to implement at least for me.

I checked the following solutions.
1. https://www.ads-software.com/support/topic/218965
Did not work for me

2. https://www.ads-software.com/support/topic/43071
The plugin by Kafkaesqui worked but it jumps between parents not children. However, in the same post, the user apeatling has modified the plugin to let children have previous next navigation. Unfortunately, when I try to download the plugin or view the source could I get error 500.

3. I landed on this page https://www.ericmmartin.com/conditional-pagepost-navigation-links-in-wordpress-redux/
Which explains how to do it but it didn’t work for me for some reason.

4. The function wp_link_pages() has become deprecated https://codex.www.ads-software.com/Template_Tags/link_pages
and the solutions did not work anymore.

I am kinda lost here. It is very easy to navigate through posts but how can I achieve this navigation in children in pages. I love WordPress. Please help me.