"What's the big deal with having Jewish grandchildren? If there is no God, or if there is a Deistic God but no Yahweh, isn't Judaism simply a lifestyle choice? And shouldn't grandchildren (and children!) make their own lifestyle choices? If my grandchildren are relatively happy, I'll be happy."
Being part of a tradition involves passing on what has been received. We, as a People, have a responsibility to ensure that our own unique ideas, history, cultures, philosophies, our whole heritage really is preserved in the hearts and minds of our descendants. To fail to do so is to doom all that we have produced - and that which has produced us - to obscurity.
No other people is going to do it for us.
Of course we must respect the wishes of individuals, even if they are our direct descendants, but a proper education can invest in them a love and appreciation for their heritage that unaided independent study will hardly ever do.