search within subcategory

I’ve searched the plugins lists and cannot find this. I’d like to put a search box in my sidebar that automatically lets readers search by the subcategory they are in. Is there a plugin for this, or do I need to hardcode in my templates?

For example: http://danceinthefire.com/category/experts/ In this overall category (“experts”), I’d like posts in the child category of “Autism Recovery” to have a sidebar search form that searches within Autism Recovery only.

I don’t want a drop down list of categories and I obviously would prefer the simplest solution with least amount of work.

Any help appreciated…Thanks!