I read that Rebbetzin Weinberg a”h of Aish HaTorah lit 80 Shabbos candles to include all her grandchildren. Is there a source for a minhag of lighting extra Shabbos candles for one’s grandchildren and may I adopt such a practice as a way of thanking Hashem for my grandchildren as well as to daven for them by licht bentshen?
It is great to be so thankful to Hashem for each of one’s grandchildren, however this is not the minhag in Klal Yisroel. If this is what she felt on their own, and it came from within her, that is one thing, but to copy another person’s madreiga is not a good idea. What you can do is to make a list of each of your grandchildren, and after you light your candles, thank Hashem for each one, and daven that each one of them should overcome thier difficulties in school, etc. and grow up to be a talmid chacham and a yerei shomayim. As a side point, the idea of lighting an extra candle for each child is a form of prayer for each one that each one should become a talmid chacham, (and the girls should marry talmidei chachomim). It is done for children, but not for grandchildren. to do it isn’t wrong, however in general we like to stick to the mesora and the accepted minhagim of Klal Yisroel
May Hashem help that all of your grandchildren should grow to be talmidei chacomim.