If you've overridden the default taxonomy page with views, you can edit the view and change "Content: Has taxonomy term ID (with depth)" under contextual filters in the advanced section.
Depth modifier description:
The depth will match nodes tagged with terms in the hierarchy. For example, if you have the term "fruit" and a child term "apple", with a depth of 1 (or higher) then filtering for the term "fruit" will get nodes that are tagged with "apple" as well as "fruit". If negative, the reverse is true; searching for "apple" will also pick up nodes tagged with "fruit" if depth is -1 (or lower).