RebE: ask your father and he will tell you, go to the zaida, my father and he will answer you.
teaching grandkids Torah gives a person olam haba .. why? the father can teach simply by example and by words. The kid will understand. The grandfather is too far away from grandkids in his experiences (do your grandkids always get your stories about Communist Hungary?). So, grandfather can’t just show and tell. So, he has to elevate the topic and explain the concept in general terms so that he and the grandkid have a common language. So, if the grandfather is able to learn like that, he gets olam haba.
what your Rebbe seem to have joked about is that in most cases there is no such direct transmission – kid asks father, who goes asks zeidi, and then explains to the grandkid. Sholom bayis, everyone does what they supposed to do, but no guarantee of higher learning and olam haba. Go speak to the grandkids directly. Well, RebE, you are talking to many of that generation here ..