Why does the Torah see fit to mention "O Ben Yigach O Bas Yigach"?
Rashi and Ramban #1: Because, since in Pasuk 29, the Torah wrote "ve'Heimis Ish O Ishah", 1 we might otherwise have thought that if the ox kills a Katan, the owner is Patur. 2
Ramban #2: Because when an ox kills a grown-up, it is wicked (like a wild bear). Consequently, if the owner has been warned three times and allows it to happen a fourth time, he deserves the death-penalty with an option of paying Kofer. Not so children, who tend not to be afraid of them and for the ox to kill a child is more of a natural occurrence; we might have therefore thought that the owner is Patur. 3
Mechilta: From "O Ben Yigach O Bas Yigach" we include a. a Tumtum and an Androginus; 4 b. Even one's own son and daughter. 5 c. Geirim. 6
Oznayim la'Torah: According to R. Akiva, 7 the Torah needs to insert this, to explain that, despite the discrepancy between the 'value' of a Katan and a Ketanah a grown-up 8 , the owner is obligated to pay his 'value to atone for his soul...
As Rashi explains (regarding a case of murder) in Vayikra 24:17 (Ramban).
Seeing as oxen do not generally attack people.
A Safek Ben or Bas (Torah Temimah).
Even though he will ultimately inherit them. See Torah Temimah, note 233.
The ox is stoned even if there are no heirs to whom to pay (Torah Temimah).
Five Shekalim against fifty Shekalim.
Why does the Torah begin the Pasuk with the word "O"?
Ramban #1: It means literally "if it (the ox) gores a man, or a woman, or a son (a Katan) or a daughter (a Ketanah) ... ".
See also Rashi in Vayikra, 4:23. Rashi in Iyov, 42:8, translates "O" as 'Asher'.
As we find in later in Pasuk 36, and in Vayikra, 5:2. The Ramban however, rejects this interpretation together with the references. See Rav Chavel's footnotes.
Why does the Torah repeat the word "O Bas Yigach"?
Bava Kama, 44a: Since the initial Pasuk is discussing a MU'ad, and where the victim died, it incorporates a Katan (who has been gored) a. by a Tam and b. by Nizakin (where he is wounded but does not die). 1
What are the implications of "ka'Mishpat ha'Zeh"?
QUESTIONS ON RASHI
Rashi writes that the current Pasuk renders the owner Chayav even if it killed a Katan. Why do we thhen need a Pasuk to render him Chayav for killing an adult?