What exactly is the Mitzvah of "Velimadtem osam es B'neichem ... "?
Seforno: It is an obligation to accustom one's children to keep the Mitzvos.
Torah Temimah (citing Kidushin, 29b): It is a Mitzvah to teach one's sons 1 Torah.
Torah Temimah (Ibid.)But not one's daughters, since the Torah writes "B'neichem"
Why does the Pasuk add the words "Ledaber bam"?
Rashi: To teach us that, the moment a child is able to speak, one should teach him to say the Pasuk "Torah Tzivah lanu Moshe ... ". And from here, Chazal learned that as soon as the child is able to speak, his father should speak with him in Lashon ha'Kodesh and teach him Torah. 1
Rashi: Otherwise, it is as if he had buried him.
Having already stated "Vedibarta Bam" in Devarim (6:7), why does the Torah repeat it here?
Ramban: In Va'eschanan, the Torah is referring to a person himself speaking Divrei Torah at all times wherever he is, whereas here it is referring to teaching one's children to do likewise. 1
Refer also to Devarim 6:7:1:2*.
