Show related post by Categories but not the Parent categories

On my site we have the main category blog (to display all the blog in a section of the site) that splits up in different sub sections: blog -> news, blog -> personal stories, blog -> events.

I would like to use the related post by category only for the child category, is there a way to exclude the parent category?

Keep in mind that for some reason (for the functionality of another plugin) when selecting a category I have to explicitly have the parent category and the child category checked in my categories when I publish a new post.