Here are a few reasons, why use a Child Theme

• If you modify an existing theme and it is updated, your changes will be lost. With a child theme, you can update the parent theme (which might be important for security or functionality) and still keep your changes.
• It can speed up development time.
• It’s a great way to get started if you are just learning WordPress theme development

How to create a WordPress Child Theme

It’s really easy to create a child theme. So let’s do it.

1. Create a Child Theme
   Open folder main_files (downloaded from themeforest) and unzip gdaynews-child.zip to ../wp-content/themes/ folder.
   
   
   You should see something like this in your ../wp-content/themes/ folder.
   
2. Create important files within Child Theme folder
   You will have to create some files within it’s folde called functions.php and style.css. Each of these files have a different purpose.
   
3. File functions.php
   After you have successfully created file functions.php, you are able to overwrite main theme’s functions.php now. HINT – for PHP customization, use a Child Theme 
4.  File style.css
   After you have successfully created file style.css, you are able to overwrite main theme’s template.css now. But we recommend doing it via our theme options in wordpress administration.HINT – for some minor CSS changes, use Theme Options
   
5. That’s It 🙂
   So the last step you will have to do is activating a Child Theme in wordpress administration Appearance->Themes.
   
   

How to overwrite main theme’s files

If you want to overwrite some files from main theme, e.g.

../themes/gdaynews/templates/custom-posts/single-gallery.php

you will have to create the same structure in child theme

../themes/gdaynews-child/templates/custom-posts/single-gallery.php

Now you are able to overwrite file single-gallery.php from main theme 🙂
Same step for plugins.

For more information, please follow this url

Biography

Child Themes. In: [online]. Available from: http://codex.wordpress.org/Child_Themes#Why_use_a_Child_Theme.3F