WordPress uses pages and posts for the main content of your site. Each page/post creates a place where visitors can go on your site. But ProPhoto templates provide the look, layout, and extra widget content for the pages of your site.
There are three ways to apply templates to the pages on your site:
Apply templates with the on-boarding wizard
Apply templates to page types
Apply a template to a specific page
Currently, custom child templates are not an option for single post pages. You can select a template for all single post pages but you cannot override the template for a specific post like you can for a specific page.