Taggy makes your Evernote tags function as you want them to: hierarchically.
Search YouTube for "Taggy for Evernote" to see a 5 minute video demonstration of this app, or simply click the support link on this page.
For many people, it's annoying that searching for a tag in Evernote does not find notes filed under child tags. Taggy solves this problem. Essentially, Taggy iterates over all of your notes, looks at their tags, and auto-tags them with their parent tags as well.
Example tags: food -> dessert -> donuts
Clicking "Hierarchify" will change notes tagged with "donuts" to be tagged with all three: "food, dessert, donuts." Clicking "Un-Hierarchify" will change notes tagged with "food, dessert, donuts" to be tagged with just "donuts."
Using this tagging scheme which Taggy creates for you automatically, organizing your tags into a tree structure will make them act that way during searches.