Is there anything wrong with a katan (child) 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?)
The only reason I could of is that it's not a "kavod" for the tzibbur for a katan to receive this honor when there are gedolim there that have a chiyuv in mitzvas.