Q. Can you honor a minor with pesichat Aron Hakodesh? (As in Avinu Malkenu, Shema Kolenu or Shir Hamaalot, or when someone else takes out the Sefer Torah)

A. Mishnah Berurah (147: 29) maintains that it is improper to have a katan carry the Torah. However, when just pesicha is involved it may appear that it should be permitted.
On question 639 in regards to giving pesicha to one preparing himself to become a Ger and already enrolled in a learning program, in contact with the Beis Din and who attends regularly a Shul, we wrote; Chashukei Chemed (Yuma 4a) rules that on the onset a prospective ger should also not be honored yet with pesicha (opening) of the Aron Hakodesh although the Rambam and others would permit. Besides the issue of Kovod or honor of the Sefer Torah involved there are other concerns such as Kovod Hatzibur and the fact that he may be equivocally accepted as Jewish even before he has fulfilled entirely his conversion (Aterez Paz O.H. 2).
You may therefore argue that it is not Kovod Hatzibur to give pesicha to a katan, when there is no established minhag to do so in that congregation.
Horav Shlomo Miller’s Shlit’a opinion is similar.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a