This is now supported as per this change by @sam
https://github.com/discourse/discourse/commit/df8f365d999e113615c51efee4cfa3115c267783
So:
To search #howto and all sub categories use
something #howto
OR
something category:howto
To restrict it so you only search the category #howto without children use:
something #=howto
OR
something category:=howto
We still need to decide what to do about the “search this category” checkbox.