You carry a burden that is doubly heavy, for not only are you separated from your grandchildren, but your son has cut himself off from you.
But don't add to your burden by blaming yourself for everything that has gone wrong. You may feel you weren't perfect parents–but none of us are, and from what you say, you tried to bring your son up to do what is right. Your son alone is responsible for his rebellious attitude, and you should not take upon yourselves a burden of guilt that you don't deserve. The Bible says, "The soul who sins is the one who will die. The son will not share the guilt of the father, nor will the father share the guilt of the son" (Ezekiel 18:20).
Can you do anything to change this situation? I don't know–but I encourage you to try. Let your grandchildren's mother know (perhaps by letter) that you deeply regret what your son did, and that you will always care about her and her children. Perhaps an occasional card or small gift (e.g., on their birthdays) might crack open the door to further contact. And pray often for them.
Most of all, I urge you to turn your burden over to the Lord. God loves you–and He also loves your son and grandchildren. Jesus' invitation is for you: "Come to me, all you who are weary and burdened, and I will give you rest" (Matthew 11:28).