Where does the custom of Kol Hanearim, were all children stand near the person getting an aliah and are covered with a tallis-canopy, come from? What, if anything, is it a segulah for? Why is it done on shmini atzeret/simchas torah?
During Simchas Torah we have the minhag of giving everybody an aliyah. Most shuls will call up boys under bar mitzvah who are old enough to know what thay are doing/ The final aliyah is considered a zechus because the person says the bracha with all of the children around him to show that they too are included in the mitzvah.
By this, I mean that he is introducing the children to the Torah and impressing them with importance of the Torah. He is showing them that they are part of the kahal and that they too can be part of the inheritence of the Jewish people. That is also part of the reason that we repeat the bracha of Yaakov (Hamal'ach hagoel ... ) The person who brings them to the Torah and the bracha of Yaakov can be considered to have a segulah of meriting children and grandchildren just as Yaakov Avinu did (as shown in the quote below).
Another reason is that there are statements that someone who brings bracha on someone else can then receive that bracha. For example, someone who arranges shidduchim for others merits his (or her) own children finding shidduchim. Someone davening for refuah of someone else receives a refuah. Here bringing the children under the "chupah" of Torah merits his own children and grandchildren being brought under the "chupah" of Torah (and of course he has to have some for this to happen).
In Kol Hanearim under the Talis, he actually shows them that they are as dear to him as his own children.
Another point is that it is similar to the mitzvah of Hakhel in which the children who are too young to understand are brought as well. This last is a point that I thought of myself as I have not seen it mentioned. The similarity just struck me.
The following is an exerpt that illustrates this point. Follow the link for the whole article.
One year, however, the auction for Kol Hanearim in my synagogue was unusually competitive. When finally over, I asked the man who fiercely bid the highest, why he vied for this honor.
Surprised by my question, he replied as if it were self-evident: "The one who supervises scores of little children crowded under the tallit, reciting the same blessing Jacob uttered over his grandchildren, is himself guaranteed Jewish grandchildren. Could I want less for myself?"
These words come to me again and again, whenever I contemplate the unique Torah portion, Vezot Haberakha, the only parasha not identified with a specific Shabbat. Rather, it is reserved for the joyous Simchat Torah holiday, with its unique Kol Hanearim ceremony, and as such deserves close analysis.
The Talmud, in Sukkah 42a, referring to Vezot Haberakha, provides a provocative comment: "Our rabbis taught: A minor who is able to speak, his father must teach him Torah.... What could be meant by Torah? Rav Hamnuna replied, the Scriptural verse [Deuteronomy 33:4], 'Moses commanded us a law, an inheritance of the congregation of Jacob.'"
Certainly these lessons are themes that the beautiful Kol Hanearim ceremony emphasizes. First, each child has a right to Torah, an inheritance that comes with birth.
Second, Kol Hanearim suggests that Torah requires effort. Neither children nor adults will acquire knowledge unless they work at studying Torah. If they put in the effort, they will be rewarded with the greatest gift: the Torah itself.
And, finally, we must appreciate that a Jewish life must include the community of fellow Jews. The little children are blessed as part of an entire group -- part of a future community -- because Torah can't be lived in isolation. Instead, our blessing emphasizes the need for everyone to be involved with the Jewish community, for only together do we comprise the congregation that both Vezot Haberakha and Simchat Torah celebrate.