At the age of 7 years old I visited Israel with my parents. Being traditional, we were unaware of the prohibition of going up to the Temple Mount. Do I have to repent for having walked on the Temple Mount as a child.
You do not have to repent for this.
Sources: See Mishnah, Keilim 1:8; Rambam, Bias Mikdash 3:4, Beis Habechira 3:15, which clarify that the prohibition of ascending the Temple Mount applies only to those defiled by impurity that emerges from body. This does not apply to pre-pubescent children. Authorities of previous generations, such as Maharit, Radvaz, and others, discussed the possiblity of entering the Temple Mount, and its borders. Modern authorities, however, including Minchas Yitzchak, Tzitz Eliezer, Yabia Omer, and others, have prohibited all entry of the Temple Mount, out of concern for entering the area of the Temple itself. According to Rambam, as ruled by Magen Avraham (561:2) and Mishnah Berurah (561:5), doing so is a heinous sin.
In the case of the question, it is therefore not clear that any offense has actually been committed. In spite of entry into the Temple Mount area, once cannot know if there was entry into the are of the Temple itself, which would be forbidden even for a child.
Concerning sins that a person committed as a minor, see Rema (Orach Chaim 343), based on Terumas Hadeshen (62), who rules that although there is no obligation of repentance, it is worthy to accept some aspect of repentance for the misdeed. Bach and Shulchan Aruch Harav (343) write that this applies to all types of sins, though Beis Yaakov (3) suggests that repentence is only required when the minor was old enough to be conscious of the misdeed. See also Shevus Yaakov (I, 177), and see Chinuch Yisrael, chap. 3, note 20, at great length.
In our case, because the concept of repentance for a minor is “worthy” but not “obligatory,” the doubt over whether any transgression took place would be sufficient to defer the need for repentance.