Child before the age of Mitzvos:
A child was below the age of Bar/Bas Mitzvah is exempt from the mitzvah of honoring his parents just like he is exempt from all mitzvot of the Torah. However, some Poskim rule that although a child is not punished for not doing this mitzvah, just as he is not punished if he doesn’t fulfill other mitzvah’s, nonetheless, he is obligated to honor his parents, and the parents and teachers must educate him to do so, and so is the practical ruling and custom.
Does one need to repent for his lack of Kibbud Av Vaeim that he transgressed when he was a child?A child [below Bar/Bas Mitzvah] who hit his father or committed other sins while still a child, although he does not need to repent when he grows older, nevertheless, it is proper for him to accept upon himself a certain matter for the sake of repentance and atonement, even though he had transgressed before he became of punishable age. [The purpose of this repentance is not for the sake of removing Divine retribution, being that he is not liable for any punishment. However, the forbidden act, despite not being punishable, leaves a damaging impact and spiritual stain on the soul of the child. It is for this reason that the child should do some act of repentance for the sake of cleansing his soul of any spiritual blemish.]
Is a person who hit his parent when he was a child below the age of Chinuch encouraged to perform a certain matter for the sake of repentance?
Some Poskim rule that the above Teshuvah is only relevant for children who damaged when they were of age to remember what they did. If, however, they were of very young age, and they don’t remember the incident, they are not required to do Teshuvah at all. However, some Tzadikim did Teshuvah even for pain that they caused their parents prior to the age of Chinuch.
 See Encyclopedia Talmudit Erech Kibud Av Vaeim Vol. 26 p. 425
 Minchas Chinuch Mitzvah 33; Minchas Soles 33:1
 Sut Bashamayim Rosh 326; Kisei Deharsana 73; See Maharsham 2:224-18
 Admur 343:11; Rama 343; Mahariy 62; Bach 343; Shvus Yaakov 1:177 brought in Pischeiy Teshuvah C.M. 349:2; Gilyon Maharsha Y.D. 240:1; Based on story in Shabbos 56b; Baba Kama 98b, brought in Kaf Hachaim 343:32; Pesakim Uteshuvos 240:1 footnote 25
Other opinions: Some Poskim rule that a child is not required to repent at all for his wrongdoing even when he becomes older. [Beis Yaakov 3, brought in Shvus Yaakov ibid] Practically, we do not rule like his opinion. [Shvus Yaakov ibid; Kaf Hachaim ibid]
 Bach 343: The verse in Mishlei 19:2 states that “Even without knowledge, a soul is no good.” This teaches us that even a child that transgresses a sin without knowledge causes impurity to his soul, it is thus proper for him to do Teshuvah; See Terumos Hadeshen 2:62; Likkutei Sichos 14 p. 144 footnote 13; Sichos Kodesh Balak 5730; Shulchan Hamelech 343
 Sefer Chassidim ibid “However if he was very young when the action was committed and he does not remember, he does not need to pay”; Beis Yaakov 3; See Piskeiy Teshuvos 343:14; Lehoros Nasan 4:34; 6:102
 Maor Vashemesh Parshas Maaseiy and Divrei Chaim Parshas Ki Savo regarding Reb Meilech Milizensk that he repented for paining his mother prior to birth and when nursing