This is so strange. I agree with others who've said that family is a packaged deal and I would not go along with arranged visits for my children.
I might find a counselor or a mediator who would be willing to work with you and your parents to resolve the issue. I'd go to the counselor for myself first with the hope that my parents would be willing to talk about his with a third party.
I had a similar issue with my daughter. She didn't want me to be involved in her life and wouldn't let me see the grandchildren. I saw my counselor and was able to feel comfortable waiting it out. I didn't respond to her when she told me that I was out of her life. Painful, yes. But it all worked out. In part because of my ability to go along with her she eventually changed her mind and we've had a really good relationship since. It's been several years. It took a few months for all of us to heal before the issue was no longer important.
I acknowledged that she had the right to make that decision. I suspect that the base issue was the question of her showing that she had the power to keep my from the grandchildren.
I suggest that this is a power move on your parent's part. I suggest that if you stand firm with the fact that you're a total unit and there will be no visits unless you and your husband are involved, your parents will eventually come around to talking about the situation.
You have the power. Your parents do not. Therefore, I suggest that you find a way to give them some power. This is where counseling will help. But in the meantime, do stand firm. There are many reasons to not want your children visiting when there is a lack of respect and trust amongst the adults.
I might say yes to visits if you can arrange them to be in your home and with your presence. Or, perhaps, they're a family affair going to a public place such as the zoo or a park. I would be willing to compromise on anything that included you and your husband in the visits.
I would meet on the dates she suggests as long as I was able to stay with the children. That does give her just a bit of power. The idea is to share the power with each side having some. However, as parents, you do have the largest amount of power.
Later in regards to losing your mom. Yes, that hurts terribly. I suggest however, that this can change. It just takes time to work out relationships. She won't talk with you now. so let it go. Eventually she may come around, especially if it means being with her grandchildren.
My mother disowned me when I was in my 30's because she found bc pills in my medicine cabinet. I cried a lot but let it be. In a few months se was once more talking with me and we soon were back to a good relationship. The idea is to not fight when we disagree. Wait and let it resolve it's self in a natural way.