No, it wasn’t wrong for you to give your grandson a Bible—not at all. What was wrong was for them to embarrass you by openly criticizing you for giving; common courtesy would have rejected such behavior.
Even if he sides with his parents right now and ignores the Bible you’ve given him, who knows what might happen in the future? Someday, for example, he may face a crisis in his life, and God will remind him of your gift and use it to turn his heart and life toward Christ. God’s Word is not bound, and it is able to break through even the strongest barriers. The Bible says, “For the word of God is alive and active. Sharper than any double-edged sword, it penetrates … the thoughts and attitudes of the heart” (Hebrews 4:12).
In addition to the Bible you’ve already given, let me suggest that you give him two other gifts—not just now, but for years to come. First, give him the gift of your prayers. From what you say, he probably has very little knowledge about God; he may not even think about God very much. Pray that God will open his heart and mind, and draw him to Christ.
Second, give him the gift of your example—the example of a life filled with Christ’s love and grace. The Bible says, “Set an example … in speech, in conduct, in love, in faith and in purity” (1 Timothy 4:12).