Did Noah have any grandchildren before the flood? Genesis 10:1 et seq. discuss Noah's grandchildren born after the flood, but doesn't say they were his first grandchildren. If he did have any beforehand, then they didn't survive the flood (7:13, 7:21). Note (for what it's worth) that Shem was ninety-eight years old at the time of the flood (11:10).
Rabbi Shimshon Refael Hirsch on Noach 10:1 (translation is from the 6 volume Hirsch Chumash)
And these are the "toldos" of the sons of Noach, Shem Cham, and Yafes, Unto them children were born after the deprivation of life.
They themselves had lived before the Flood, but their children were already born under the influence of the new conditions.
There are also other meforshim which speak of them deliberately not having children before the flood in order not to expose them to the conditions and no-one (animals and men) having relations on the teivah).
Noach had also not had children until he was 500 (100 years before the flood) and he had been working on the ark for twenty years.
All this implies that Noach had not had any grandchildren before the flood. The statement in Noach 7:7
And Noach went in, and his sons, and his wife, and his sons' wives with him, into the ark, because of the waters of cessation of life.
also seem to imply that they were the only ones in that family who could have gone in. Thus, they had not yet had children.
According to the Haamek Davar (9:9), Noach had grandchildren who survived the flood: 7:1's "and your whole household" includes those grandchildren (and servants and personal animals).