Answer: The Rema (OC 669:1) writes that on Simchas Torah we add on extra aliyos, reading the same parsha a few times. The Mishna Berura (669:12) explains that we want to give everybody an opportunity to participate in the simcha of the Torah. R’ Shlomo Zalman Auerbach (Halichos Shlomo, Sukkos 12:15) adds that when everyone receives their aliya they get to recite the beracha, ‘asher bachar banu.. venosan lanu es toraso, Who chose us.. and gave us His Torah’ (See Rivevos Ephraim 2:176)
While the Mishna Berura (282:12) writes that we shouldn’t call up a child under bar mitzva for an aliya during the year, the Rema writes that on Simchas Torah we are accustomed to give all the children a joint aliya. The Mishna Berura (669:13) explains that this serves to train them in the mitzva of reading from the Torah. According to the Shaarei Ephraim (8:57) this is also to give them a love for the Torah.
The Kaf Hachaim (OC 669:4) writes that every child gets their own aliya and if the child is unable to read the beracha by themselves then the baal korei should assist them. While different shuls have different customs, R’ Shlomo Zalman Auerbach (Halichos Shlomo, Sukkos 12:n20) writes that we should only give aliyos to children when they are at least nine years old and mature enough to follow along properly. The Shaarei Ephraim (ibid.) suggests that even children as young as six or seven can receive an aliya.
In conclusion, younger children should be brought in to shul for their joint kol hanearim aliya. Older children who can read the berachos and can follow along should be given their own aliya, depending on the shul’s custom.