When should I use the child-theme?
Child-theme is something about developers. if you are a developer and need to modify the theme core files, child-theme is something handy for you. otherwise, you don’t need to use it.
If you are using Phlox Pro, please note that the child-theme is located in the "child-theme" directory inside the downloaded package from the marketplace.
For Phlox Free, please download it from here:
For more info on child-theme, please read WordPress official documentation here: http://codex.wordpress.org/Child_Themes