It is an unfortunate situation that you find yourself in, no doubt. One option would be for you to baptize your grandchild in secret, with the correct formula, and it would be valid. Now, as many people on this thread have already pointed out, doing such a thing would be against Church law. However, in these modern times, our own Holy Father, Pope Francis, has warned against “Doctors of the Law” and “legalism” on numerous occasions. So do not be too concerned about breaking a Church law if you are acting in good faith. Just do what Pope Francis would tell you to do - follow your conscience. If your conscience tells you that you should baptize your grandchild, then you should do it. Do not be like the pharisees, who were worshipers of the law instead of worshipers of God.