Sean's answer definitely works if you know the Formatted IDs of the parent folder and all of its children, but if you already know that then you could probably answer your question just using the Quality >> Test Plan page.
If I understand the question correctly what you'd really like to do is to query the TestFolder object and show all child folders and their test cases. Unfortunately the TestFolder object isn't one of the types available in the Custom List app. You can get the first two layers of your folder hierarchy (top level folder and its test cases plus the immediate child folders of the top level and their test cases) using a query string like:
((TestFolder.FormattedID = "TF1") OR (TestFolder.Parent.FormattedID = "TF1"))
There's no way in the UI to get beyond the parent folder level to grandparents, great-grandparents etc. so if your folder hierarchies are more than two levels deep then you'd need to do something custom against the WSAPI. Wish I had a better answer for you but if this is just a one-off query then you could definitely get your results using Sean's method. Hope that helps.