You cannot. Let me say that as firmly, forcefully and kindly as
possible. While you had the responsibility of being their mother, you
raised your sons and gave them your values. They are now grown men, married
to the women of their choice, and they have made a family decision on how to
raise their children. You have no responsibility in that arena, nor is it
your place to "help integrate" these two traditions.

Your job is to love these children and not to interfere with the
way your sons are raising them. The best thing you can do is to support
your sons' decisions. Nothing else would be loving.

I hope you do not experience this as harsh. The desire to have our
grandchildren reflect our values is deep in all of us. However, the idea
that having part of your family raised in a different religion from your own
is a problem only for those who believe that their religion is the only true
religion and that there is some inherent duty to "save" family members by
making sure they are also introduced to the "true faith." That attitude,
abandoned by most thinking people (still, however, in the rhetoric of both
the Vatican and the Protestant Evangelicals) is the source of great family
tension and frequent fractures.

If you love and accept your sons' decision, your daughters-in-law's
commitment to their belief and your grandchildren's religious path, you will
in fact be doing the Christlike thing. You are loving them unconditionally
just as they are. You are enhancing their lives, affirming their being and
telling them that nothing can separate them from you or from God's love.
That is the pathway for grandparents to walk. I hope for your sake and for
the sake of family peace and your grandchildren's future that you can do
just that. They will honor you for it.

John Shelby Spong