Hi, I'd like to help you with your question.
Your daughter, for now, has chosen her husband over her relationship with you. She has also decided to cut off her son's from seeing you or other family members. There could be many reasons for this.
One, she is shocked by the information about her husband and is not yet able to deal with it.
Two, she doesn't believe the information and feels she needs to defend her husband.
Three, her husband has told her he did not do this and either asked her to make a choice, or forced her to make a choice between the two sides.
I understand your concern about your grandsons. Based on your son's experience, these boys are in danger. Your daughter cannot be there with her son's all of the time. One option you do have is to check with authorities to see if charges can be brought against your son in law. This will probably not repair your relationship with your daughter but it will protect your grandsons.
If you want to repair your relationship with your daughter, you can continue to try to make contact. If you have a chance to see your grandsons, let them know you love them and care about them but there are some adult problems going on and that until all of you can work it out, things may be tough for a while. Also, let them know they can call you or contact you any time it is ok with their mother.
You can also try therapy. See if your daughter will come with you. Let her know you want to repair your relationship and having someone who is neutral in the situation will help you both work this out.
Continue to call your daughter and on holidays and other special events, send cards and invitations. Let her know that what happened with your son may be a shock to her, but you wanted to be sure her sons were safe. Let her know you care about her and welcome her and her sons anytime. But stay firm with how you feel about her husband. It is unfortunate that you must chose between your son and your daughter's husband, but for your son's well being it is important that you protect him. Hopefully, with time, your daughter will come to understand the situation and make amends.
I hope this has helped you,