We are as sick as our secrets.
You knowing about her drinking, and not being able to confront her, and not telling the girls father puts you in the position of keeping a secret..which, in turn, makes you another victim of this disease called alcoholism.
Tell the girls father, you have to be their voice in this situation. It may get really ugly between you and your daughter, but, better some unpleasant moments then something tragic happening. If she is drunk, she is not fully present to watch her children, it is that simple.
Maybe this is HP's plan...for this to come out, the girls to be safe and your daughter might finally reach out and recover.
Your grandaughters are lucky to have such a loving, caring women looking out for them.