The Rebbe fleshes this out even more. "Machar" can mean tomorrow, or further in the future, or a different mindset, a topic/mind-shift. A child can come to ask the question because hes in a totally different mind-space or mindset and doesnt "get" whats going on here. The Rebbe digs deeper: we can have 2 types of children who ask questions. One who just simply doesnt know, when we teach them, they join in, and the other who doesnt get it, they are "machar sh'l'achar zman" This child "doesnt know yosef" - doesnt know the Rebbe, doesnt relate, maybe isnt even sure if they believe what you are saying. What should we do with such a child? Talk with them, answer them, connect! Do not give up! He is YOUR child. This is your zchus and achrayus. We need to tap into an even greater love for this child!