You are wise to be concerned about your grandson's spiritual welfare (as well as that of his parents). In all likelihood, he will grow up without any understanding of God, or even of right and wrong–and that's a recipe for moral and spiritual danger.
At the same time, I know it's difficult for parents to give advice to their adult children without sounding like they are condemning them or trying to run their lives. Before you talk with your daughter and her husband, therefore, first talk with God. Ask Him to give you wisdom to know how (and when) to approach them, and ask Him also to make them receptive to what you need to say. The Bible reminds us to "pray for each other. ... The prayer of a righteous man is powerful and effective" (James 5:16).
Then share your concern with your daughter, telling her that while you don't want to interfere, you do worry about your grandson growing up in a world that has few moral standards. You may be surprised to discover that down inside she may have the same concern.
Becoming parents often makes people stop and reevaluate their lives; it probably happened years ago with you. Don't be hesitant to reflect on your own experiences, and assure your daughter of your love and your prayers for them as they raise this child.