The advice I give is advice that you probably don't want to hear. I have been the daughter in law in a situation not too far from yours.
At the end of the day, they are her children, not yours. She has the right to determine who they spend time with and who they don't. To say that your son isn't much of a man because he "lets her get by with this" is ludacris. Perhaps he agrees with her. And even if he doesn't, she is his wife, and her thoughts and feelings and opinions should, and obviously do, rank higher with him than yours do.
My situation was different in the fact that when all our arguing was going on, we didn't have children. Luckily now that we do, we have repatched what can be repatched of that tattered relationship. This started by them saying things about me and accusing me of "turning their son against them". I would definatly NOT go down that route. My husband still has hard feelings towards his parents because of what they did to me, and ultimatly him. His relationship with them will never be what it was because of the way they tried to butt into our lives and blame me for everything.
I am sure that you will get other advice, supporting you, but I just can't do that.
Good luck! I am sure you miss your grandkids, it must be tough, but attacking their mother and your son are not the way to go about getting to see them.