How far does privacy and journaling extend?

Let’s suppose a person is in the habit of keeping small journals or notebooks. While most of these notebooks are kept in personal space, one or two are often left laying out in common family space (living room coffee table, kitchen counter) for weeks at a time. A child of the journal keeper sees one of the journals, and finds an inscription in the front that dedicates the journal to the children of the journal writer. Does this dedication give the child permission to read the journal? Should the child ask permission first? Does the age of the child matter?

What if the spouse of the journal keeper opens the journal and sees the dedication? Would he/she have implied permission to read the journal? Does the expectation of privacy extend to notebooks, papers, etc. left in shared space?

Suppose the journal escapes being read, and at the time of the death of the journal writer, the spouse reads the journal dedicated to the children, and finds that while it starts out as a commentary on nature, it also contains explicit details about an extramarital affair the journal writer had while his children were young. Should the spouse destroy the diary or is he/she obligated to honor the dedication?