I would like to answer your question from a different angle.
I doubt you can change either your DIL or your son, as you cannot change other adults, and especially because you have put up with a lot of unreasonable demands from this young couple for many years.
I do understand why you feel you hate your DIL, but hate is destructive - not to her, but to you. Your DIL is not bothered if she sees you and your husband or not, nor does she apparently feel that her children should see their grandparents, or her husband his parents.
You cannot change this, whatever you do.
Your son either cannot or will not stand up to his wife. You cannot change that either.
What you and your husband CAN do is to write a letter to your son and daughter-in-law, saying kindly that you regret that the fact that you are unable to help with the child-care they need apparently has led to your not seeing them and their children at all. Add that they are welcome at any time to ring and ask if they may visit you, and if you and they can find a time that suits, they will be very welcome. The same applies if they prefer for your son and the children to visit without your DIL.
Finish by saying that you hope you all can put this disagreement behind you.
Then sit down with your husband and discuss what the pair of you want to do together in your retirement, and whether you each have separate interests.
Use this time to do things you have always wanted to, rather than centreing it on your son and his family. None of us know how long we are going to live, or be in good health, so enjoy this time, while you are still together.