I was using the "Related changes" watchlist but the changes shown are just related to the parent category and not those of its subcategories. Is there a way to include all the subcategories? If not, I request that be a feature. I use this watchlist to see changes pertaining to WikiProject Central America, so it would be great if I can see the changes of the parent category and all its subcategories.
Topic on Talk:Edit Review Improvements/New filters for edit review
It is not possible for now to search in sub-categories. There are multiple questions to solve that were too complicated, both technically (display pages from sub-categories would, in certain cases, display thousand of pages) and in term of classification (some categories groupe topics that are too different to be a relevant result).
Providing categories to Recent Changes, Related Changes and Watchlist is already documented. The current need is to have time and ressources to implement it; that feature could be suggested to the Community Wishlist survey for instance.