The only problem was the unwieldy HTML coding required to ensure that the lists opened and closed, as expected. What ended up working better for me was Microsoft Word's Outline view, which lets you use headings to show and hide information in a tree structure.
The O'Reilly family tree my cousin created has about 500 names and covers seven generations, which convert to Word's headings 1 through 7. To view the entries as collapsible and expandable lists in Word 2003 and 2007, click View > Outline. Any entry with a plus sign on the left has subentries; double-click the plus sign to view or hide the entries.
Then I took the list one step further by linking the names on the family tree to a mini "Web site" with basic biographies of family members. (For obvious privacy reasons, I'm keeping this entire family database off the public Web. The individual files reside in a single folder stored on local media, not a Web server.)
I used an HTML editor to create a page that duplicates the entries and links on the family tree in the Word file. Unfortunately, HTML supports only six levels of headings, so I manually italicized the seventh-level entries to distinguish them from the tiny sixth-generation names.