MB 11: If they have grown two hairs - See in the Magen Avraham [commentary on the Shulchan Aruch by Rabbi Avraham Abeli Gombiner, 17th cent.], who writes that according to the opinion of the Tur [Legal code on which the Shulchan Aruch is based, by Rabbi Yaakov ben Asher, 13th-14th cent.], in Even HaEzer, Siman 21, once this age is reached (12 for a boy and 11 for a girl) it even if two hairs have not grown it is forbidden to read the Shema without a cloth separating. And even without reading the Shema, it is forbidden to lie in bed together naked in close physical proximity with one's child, once he/she has reached this age, and see in the Chelkas Mechokek [Rabbi Moshe Lima, 17th cent.] and the Beis Shmuel [Rabbi Shmuel ben Uri Shraga Faibish, 17th cent.] there.
Lawton Cooper
[Today's Halacha is going to be surprising, or may even offend some readers unfamiliar with traditional Jewish practices in the area of male-female interaction. It may be disconcerting to realize that so much that is taken for granted in the 1990's is seen in Halacha as inappropriate. However, I honestly believe that a Madison Ave. advertising firm would be the first to agree that the Rabbis obviously had great perception of what excites men about looking at women, or hearing them sing - and that whereas the former group designs their material to attract mens' eyes, the latter take great pains to steer men away from the resulting temptation to be attracted to a woman, especially where a relationship with her would be inappropriate.