when user read the posts for a category A, maybe better include those posts from subcategories of A ?
So you mean when a user hovers on a sub category on the homepage, it switches the preview to posts in those child categories? Not sure teasers are included in in the child categories. Could get a bit confusing.
No -- I mean when user click on a category X and go to the page for that category -- there are 2 parts - 1) list of sub categories ; (2) list of topics for category X ( I mean should the topics include topics created in the subcategories of categoryＸ？).
from UI perspective, it's fine for homepage use lavender theme style, but for the category page with subcategories, the style of many boxes on top of list of topics seems not good.... better make the list of sub-categories very light for example just a list of text links.....