I found this article to be helpful:
I’m not a scholar and don’t know ancient Hebrew, but just a scan on Bible Hub to reference the verse I did find that the phrase ‘talk against’ (criticize) is a feminine verb, so it is possible that Miriam was the instigator.
Aaron may not have received leprocy, but God did call him out together with Miriam. I do believe God is just in His judgement, so perhaps the case in this article stands to reason. Important to note, Aaron repented and confessed the sin both he and Miriam committed when seeing the skin disease on his sister, Miriam did not. I have to wonder if God was reaching these two in ways He knew would be most impactful.
This may or may not be helpful for your lesson, but I think it is a good idea to share with young believers that we don’t always see clearly why God responds they way he does in scripture. We do see that he is involved, and in this story He does discipline Moses’ siblings for speaking against God’s servant which is an important lesson: God is involved in our spiritual growth and disciplines His children, Hebrews 12:7-8. I do not doubt that, as a loving father knows, God knew what was needed to reach these two appropriately.