Answer:
   
   There are three issues with respect to tzeni'us (modesty):
    1. The exposure of areas absolutely deemed erotic
    2. An objective standard that isn't subject to societal norm
    3. The societal standard
       
   To apply this model to real life: Nudity, such as bathing, of the
   first sort, as might be short shorts or bikinis. According to the
   Aruch haShulchan, this is banned at least by age 3 for girls and 9 for
   boys. The disparaty in age has to do with the difference in age
   required for rape to be a realistic problem. This is true for all
   people of the opposite gender, and according to this text, fathers
   included. Some authorities are more lenient, ruling that fathers are
   an exception to the three and up rule, and no prohibition starts at an
   age where the child is too young to learn about such things, just as
   in any other home.
   
   A better known case of the second category would be going sleeveless.
   Another that the Aruch haShulchan discusses is the Talmud's
   pronouncement that a woman's hair (which is understood to mean a
   married woman's hair) is erotic. This is an objective standard; it
   holds even in societies that aren't shocked by these things. However,
   it is also not blatantly erotic in the normal sense of the word.
   Married Orthodox women by and large cover their hair (or at least know
   they're supposed to). When one starts observing these depends on the
   child; i.e., when they're educable in such matters (as in any
   mitzvah). They ought to learn before reaching b'nai mitzvah age, but
   the number of years before is going to on the child. This includes
   sleeves that go past the elbow and skirts that go past the knee even
   when sitting down.
   
   However, when it comes to distraction for prayer, we go by what
   distracts -- which is going to be societally determined. So, the Aruch
   haShulchan rules that one may say Shema in the presence of a woman
   whose hair is uncovered. Societal standards, in other words, things
   that aren't blatantly erotic, aren't spelled out by halachah, but are
   considered "not done", wouldn't apply between a father and daughter
   until she is married. The word "wouldn't" is used because once we live
   in a world where Calvin Klein can put up billboards of women in their
   underwear and bikinis are acceptable, this category is empty. There is
   nothing beyond the core of the body that will shock most people today
   even when seen in someone other than one's daughter. However, if you
   still haven't lost the art of blushing... your married daugher
   shouldn't wear in front of you something that would make you blush if
   worn by someone else -- even if the area exposed isn't spelled out by
   halachah.