Grandchild nodes (of a nested taxonomy term) on node page
You can start with Content Views that show all nodes tagged with term from Vocabulary A, then add relationships to Taxonomy information, including the reference between terms from vocabularies A and B, and the one introducing content tagged with those terms. 


Create a new Views of type Content showing Fields, add a Block Display to the View, change its name to something meaningful you can recognize later on Blocks page
Add a Filter to limit to your desired Content types
Add a new Relationship Taxonomy term referenced from field_a and Require this relationship  

This one merely introduces more information about the Vocabulary A term that the current node is referencing.
Add a new Relationship Taxonomy term referenced from field_b, make it use the first Relationship and Require this relationship  

This one introduces information about the term from Vocabulary B that is referenced by the term that node is tagged with. 
Add a new Relationship Taxonomy term using field_b, make it use the second Relationship and Require this relationship  

This one goes reverse, back from Vocabulary B to terms from Vocabulary A that reference specific term from B
Add a new Relationship Content using field_a, make it use the third Relationship and Require this relationship  

This one introduces nodes that are tagged with terms from A that also reference term from B that is referenced by term from A that the current node is tagged with. 
Add a new Contextual filter ID from Content Category and configure:
When the filter value is NOT available
Provide default value
Content ID from URL
When the filter value IS available or a default is provided
Specify validation criteria
Content
[your relevant CTs]  

This one just gives Views information about the current node. 
Edit the Content: Title Field you already have and make it use the fourth relationship (it will actually be the only one available). Now the Title won't be showing the Title of the current node, but the Title of the node that is related to the current node through A > B > A "relation path".  


You should now see a list of nodes related to the current node through the "grandparent" term.