It will depend on the child, but you may want to let your DD know that this is her chance to give her GGM a hug goodbye.
I sat at my grandfather's deathbed, and although older, a nurse told me that it was my chance to thank him and tell him how much I loved him. So I did that, and though heavily sedated at the time, he tried to answer me and he squeezed my hand. So when he did pass, he knew how much we loved him.
I know its different with Alzheimers. But maybe knowing how loved you are can get through if there is even a moment of clarity left.