How get posts from a subcategory of a category by name?

I'm trying to create a subcategory template of a defined category. For example, for each one, I would like to check if his subcategory Photos exists and to get posts of this.

Here is my category tree:

— Category Sessions
— — Subcategory Session 2015
— — — Subcategory Photos
— — — Subcategory ...
— — — Subcategory Videos
— — Subcategory Session 2014
— — — Subcategory Scores
— — Subcategory Session 2013
— — — Subcategory Photos
— — — Subcategory ...
— — — Subcategory Videos

If subcategory Photos of this session category exists :

My idea is to have a query such as :

$wp_query = new WP_query(... Session 20XX/Photos ...&showposts=20);

Is there a way to do that please?