Nietzsche (tr. Walter Kaufman),
Die fröhliche Wissenschaft, V, 369:
No one knows a child less well than its parents do. (Niemand kennt ein Kind schlechter als seine Eltern.)
The reverse is also true -- no one knows a man and woman less well than their child. Add this (Nietzsche,
Also sprach Zarathustra, III, section
Von den drei Bösen, 2) and the circle of misunderstanding is nearly complete:
Who can comprehend how strange man and woman are to each other? (Wer begriff es ganz, wie fremd sich Mann und Weib sind!)