I am trying to figure out if there is a way to modify the default taxonomy view so it shows nodes tagged with that particular term (already done) plus all children of the taxonomy term you are viewing.
I have seen solutions for D7 but nothing for D8.
One way of doing this is with the use of https://www.drupal.org/project/eva which lets you attach any entity to any other entity
Just for demonstration I have setup
parent 1 child 1.1 child 1.2 parent 2 child 2.1 child 2.2 install and enable eva create a taxonomy terms view called children with no block or page display
Then add an EVA views display from the dropdown You will need to set a contextual filter parent term with following settings
Hide when value not available specify validation criteria taxonomy term id: [NAME-OF-VOCABULARY] hide view 
and on entity content settings set entity type: taxonomy term and bundle: [vocabulary name]
I had to clear cache so it does not hurt to do it and visit any of the taxonomy parent pages you should see the children terms
you can change where the children are displayed at your taxonomy term display settings page
