Filter by taxonomy parent using Json api
The term-ID 2 belongs to "Term B", not to "Term 4" or "Term 5". To check if term belongs to Term B you may include the term parent and check if it has id 2:

https://www.some-api.com/jsonapi/node/test?
_format=api_json&amp;
include=field_regio.parent
filter[taxFilter][condition][path]=field_regio.parent&amp;
filter[taxFilter][condition][operator]=IN&amp;
filter[taxFilter][condition][value][]=2 (=&gt; id of term B)


This should work at least for the simple example provided it the question.