customize navigation / sub-navigation of wordpress theme

I need my wordpress site to list the child categories when on the parent page, as well as showing the specific child category as selected when on the child category page...Each parent page will have upwards of 6-8+ sub-categories, and these sub-categories will be different for each parent....so using some sort of call like: $categories=get_categories('child_of=3'); foreach($categories as $category) { to at least list the categories, but then those sub-categories need to be links.....