Hello,
You have by default a module that can quick display on 2 levels, you can find it in :
1. Extensions => Modules
2. Then look for "Categories on 2 levels"
3. Click to edit on the name
4. Go in HikaShop Options tab
i.imgur.com/TZHGpdw.png
5. Select your parent category :
- Main parent category:
Parent categoryA
category child1
category child2
category child3
Parent categoryB
category child4
category child5
category child6
Here, in Main category to follow my idea, you have to select"Main parent category".
Hope this will meet your needs.
Regards