How to specify child categories?

I’ve successfully set up Postie but I can’t work out how I would specify child categories in the subject line. I’m fine with the parent level categories but what is the syntax for a child category e.g

parent category
– child category 1
– child category 2
other parent category
– other child category 1
– other child category 2

So this email subject works fine:-
[parent category] This is the post title

But how do I get the equivalent of this:-
[parent category > child category 2] This is the post title