Applying templates to terms doesn't work with child terms

I have Category A and OTHERS. All have child and grand-child terms.
I need a template for ALL categories EXCEPT A (and its subs).
I make a template (Main-Temp) → set conditions for POSTS and EXCLUDE A
Then make a template (A-Temp) → conditions for POSTS and INCLUDE A
A-Temp works for posts in the main category, but not for child cats.
Main-Temp takes over again for all child categories of A.
The client needs to be able to add categories under A → and not have to call me to add every single category to Include on A and Exclude on Main.

How do I apply a template to all children?