Q. A grandfather and his son who has his own children are at the Seder together. Who recites the Haggadah?
A. The mitzva of Sipur Yetzias Mitzraim or telling the Haggadah story applies to a grandfather also. (See Talmud Kiddushin 31a in regard to the mitzva of learning Torah that it applies to grandchildren also). Teshuvos Vehanhogos (2: 236) maintains that it is even a greater mitzva for the grandfather to tell the Haggadah to the children.
Chassam Soffer’s opinion is that the grandfather should tell his son, and the son then should tell his children
It would seem that the best way is to divide time. The father should explain shortly to his children the answer to their four questions. The rest of the Haggadah that as is is hard for them to understand and keep attention to, should be recited by the grandfather that as is is leading the Seder.
A father complies with this mitzva even if his children area grownup and also already Talimidei Hachamim. (Rambam H. Chometz Umatza 7:: 2-3).
Horav Shlomo Miller’s Shlit’a opinion is similar.
Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a