The WP Use parent template didn't work as expected.
When the child category query is redirected to parent category template, the child category query is replaced by a global query. Oops, wrong behavior.
I'm still trying to find a plugin that does something obvious: uses the parent category template for child category queries. This would save me some elaborate conditional coding on the category page, or save me the horrid redundancy of multiple nearly-identical category templates for each category.