What does "Dodaso" mean?
Why does it say "Aririm Yamusu", and in the coming Pasuk "Aririm Yihyu")?
Rashi: If they already have children, the children will die, and the parents will die childless. If they do not have children yet, they will remain childless.
Bearing in mind the Pasuk "Ish be'Chet'o Yumasu" (Devarim 24:16), how can "Aririm" be telling us that children who are already born will die for the sins of their father?
Gur Aryeh: Only children who are minors at the time of the father's sin will bear the punishment of the father's sin, since we find that minors are indeed punished for their father's sins (see Shabbos 32b, Kesuvos 105b, Rashi Koheles 5:5)
Panim Yafos (24:16): If the child sins b'Shogeg, but the father performed the same sin b'Meizid, then the child will be held accountable as if he too sinned b'Meizid even if he is grown up, because of his father's ill intent. But if the child did not sin at all, he will not be punished for his father's sin.
Why does the Torah insert "Aririm" specifically by Dodaso and by Eshes Achiv (in the next Pasuk)?
Ramban: Because it applies exclusively to them. 1
Rashi (17:9) writes that Ariri (being childless) applies to all the Kerisos. If so, why was Dodaso repeated? We already know that there is Kares from 18:29! (Moshav Zekenim 17)