You can right-click Christmas in the Keyword List panel and turn off the option "Export Containing Keywords". This will suppress the inclusion of Events and Events > Holidays in XMP:Subject and IPTC:Keywords, but still include Events|Holidays|Christmas in XMP:HierarchicalSubject.
Unfortunately, there is no way to batch-edit all of your keywords to turn off Export Containing Keywords -- you have to go one at a time. People have requested that ability for years in the official Adobe feedback forum, but Adobe hasn't responded. And a minor bug in the LR Software Development Kit prevents plugins from providing the missing functionality. People have posted recipes for using low-level database tools to modify the LR catalog, but that's obviously difficult and fragile.
Alternatively, you could turn off the option "Include on Export" in the parent keywords Events and Holidays. This will also suppress their inclusion in XMP:Subject and IPTC:Keywords. But it will also suppress their inclusion in XMP:HierarchicalSubject -- you'll get Christmas rather than Events|Holidays|Christmas. But you may find that a better compromise.
Hmm. Thanks for the clarification, John. It looks as though the workarounds have their own limitations. I might just see what the result is of having LR throw all the extra levels of keywords into dc:subject. It might break something else in my workflow, but I'll give it a go. Thanks.