Allows theme authors to add page templates to their themes that will be automatically used based on the page's parent.

Noted That:

To install correctly this child-page-templates.zip.
Fisrt Download the child-page-templates.zip to your computer
Extract/Open child-page-templates.zip to Your Computer.
Then, Find readme.txt file inside child-page-templates.zip and Open readme.txt.
Now, Read the Requirements of this plugin. Which Wordpress Version and PHP Version are required to run this Plugin in Your Wordpress Site.
Then, Follow the the Tips Below.

Start the Tips:

1. First Download "child-page-templates.zip" Plugin to your Local Computer. (Click Download)

2. Then, Login to your "yourdomain.com/wp-admin" Dashboard.

3. Then, Click on "Plugins" + "Add New" from left sidemenu of Dashboard.

4. Now, Click on "Upload Plugin" button.

5. Now, Browse "child-page-templates.zip" Downloaded plugin from your computer, Where you downloaded child-page-templates.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "Child Page Templates" folder is added on left sidemenu. Now, Click on "Child Page Templates" folder.

Noted that: If you do not see "Child Page Templates" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about Child Page Templates Configurtions and Settings or How to work "Child Page Templates" in your WordPress site.

After Activated Plugin According to Step-6 then,

Go to "Plugins" + "Installed Plugin" from Wordpress Admin Panel Leftside Menu.
or Direct go to: https://yourdomain/wp-admin/plugins.php
Then, Find "Child Page Templates" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is Child Page Templates
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide
Extract the zip file and drop the contents in the wp-content/plugins/ directory of your WordPress installation and then activate the plugin from the “Plugins” page. Create a folder in your theme (or child theme, if you use one) called “child-page-templates” Add a PHP file named after the slug or ID of the parent page you want to use a custom template for. For example: If you want every child page of your “Portfolio” page to use a particular template file, name that template file “page-portfolio.php”

Guide

Extract the zip file and drop the contents in the wp-content/plugins/ directory of your WordPress installation and then activate the plugin from the “Plugins” page.
Create a folder in your theme (or child theme, if you use one) called “child-page-templates”
Add a PHP file named after the slug or ID of the parent page you want to use a custom template for.
- For example: If you want every child page of your “Portfolio” page to use a particular template file, name that template file “page-portfolio.php”

used, automatically, based, pages, parent, themes, their, theme, authors, page,

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Reffered: https://wordpress.org/