Hi R.,
That is such a tough situation to be in. You must be so worried since the grandmother can't keep safety if she's in denial. I assume there is a social worker involved? If so, tell him/her your concerns and ask what safety measures are in place. If the other grandmother is getting visitation, she must have done some work to show social services or whoever that the children can be safe with her, i.e. no strangers can be left alone with the kids, etc. Also, I assume Victim Witness is involved if they aren't already. They can help with counseling and such.
As for the kids, you don't say how old they are, but in a way that's appropriate for their ages, make sure they understand that regardless of their grandmother's denial you are there for them and they can talk to you. Make sure they know you believe them, which will add to their feelings of safety. And when they return from their visits w/her, just gently check in with them. As much as you may want to, don't grill them for info or push too much. In a light-hearted way, ask general questions like "How was your visit? What did you do? Did you have fun? Did you feel safe there?" And for every positive, let them know you are glad.
I would be totally stressed too, but in such situations we have to surrender some control and trust that things will work out, while at the same time keeping an eye on your grandkids and making sure you and your home are safe for them.
Good luck. I'm sending up prayers for you all.
L.