Telling the children their Grandma is dying

My DD’s MiL is very sick, and liable to die in the near future. This has been fairly sudden - although she was diagnosed with cancer a couple of years ago, things have suddenly deteriorated. DD is very anxious about telling the boys, especially as the nearly-6year old is very anxious at the best of times - having nightmares about the sun exploding and killing us all! They don’t see their GPs very often as they live some way away. DOes anyone know of a good book or other way of explaining what is happening in a way that won’t freak DGS out?