It is a delicate question. It depends on where the wedding will take place. If the wedding is in another Christian Church, let us say, in an Orthodox church or a Protestant church, I think the parents can assist and take part because it's a Christian ceremony. I just hope that there is a dispensation from the bishop from the canonical form.
If it is not a religious wedding, but only a civil marriage, I think in this case it is not permissible because it is only a civil marriage. For us Christians and Catholics, it's not valid, it is against the law of the church because the church is not recognized in civil marriages.
I will say that they can in some way take part in their children’s lives and show their love for them but they are not to take part officially in the concluding act that the marriage was civil.
Maybe they can separate these acts and say, “Okay, we will help you, even with material gifts for your new life. We recognize you as my child and we would often come to visit you and be in contact with you and we love you. But because of our conscience and in our faith, we cannot assist in the act of civil marriage because it is marriage without the consideration of the Catholic faith.”
Every case is different but in general, I would say that the parents have to demonstrate their love even for these children and their offspring who are living their marriage not following the Christian faith.
Sometimes with love and patience, maybe we can conquer their hearts so that after some years they might return to the church. So please maintain contact and love for your children and pray for them even when they are heading the wrong way.
Dear visitors of Gloria Dei and all who have already written or who will write a personal message to me. I am grateful for your attention, your kindness, for your spiritual support and especially for your sacrifices and prayers for me. God may reward you!