Is there anything wrong with a katan receiving the honor of opening up the aron kodesh (for example when saying Avinu Malkeinu, or during Selichos, or after davening for Anim Zmiros, or even when to take out the Sefer Torah — however perhaps the katan would just open the aron and someone else would take it out — or perhaps even the katan would do this?)
It is improper for a child to hold a Sefer Torah (Mishnah Berurah 147:29).
It is permitted in theory for a child to be honored with opening the Aron Ha-Kodesh.
However, in practice this is not the general custom. It is a worthy practice to reserve the honor for adults, so that it will receive the deserved respect. However, where it is custom to give a child Pesicha (for instance, for An’im Zemiros), this is fine.