I'm very sorry to learn of the difficult time your daughter (and your grandchildren) appear to be having. If your description is accurate, I can understand why you are so concerned.
Unfortunately, there is nothing that you - as mom or grandma - *can* do that will help "her understand what damage is caused to her and is causing to her children..." I know that this is what you want for her, and this may be what she herself wants - but there is some reason why she isn't willing to leave or insist that her husband gets help.
In the event that your son-in-law becomes violent, I strongly encourage you to contact the police. Yes, this will likely be very uncomfortable for everyone involved, but there is a point (physical violence or violating the rights of others) at which legal authorities need to get involved for the safety and well-being of the children (and your daughter). If it has not risen to the level of violence, yet you have significant enough concerns, you may wish to contact your local Department of Children and Family Services and make a report of suspected abuse or neglect. (Please understand that such reports typically remain anonymous... and may result in a number of possible responses from the agency, for a host of reasons, ranging from "nothing" to removal of the children.)
It may be worthwhile to encourage your daughter (and children) to pursue family counseling on their own. While this may not solve the problem, it will allow them an opportunity to express their fears (and other feelings) and also to develop strategies to cope with their home environment. Working with a licensed mental health professional (LMHP) who understands family functioning will be crucial.
Lastly, I might recommend for YOU (and possibly your daughter), Alan Kemp's book, "Abuse in the Family: An Introduction." It may or may not be appropriate for you, but I have heard it is well-received among practitioners and families.
I wish you and your family all the best. Thanks.
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