This was a big help, @dim, thank you!
Now I think that I’ve gotten to the problem, but I don’t know how to fix it.
When I apply the default theme, my “Secondary Widget” content moves to the “Main Widget” container. When I move it to back to the “Secondary Widget” container where I want it, the content displays correctly in Safari. When I reactivate my Child Theme, however, the content in my Secondary Widget area is deleted.
Any ideas?