So sorry you're going through this. My ds's nan (my mil) died last year when he was 8.5 yes, his papa died four years before. I dealt with both differently.
Mil had been noticeably ill for a while and because we didn't know how long she had, we waited until we were told she had weeks. We decided i would be the best person to tell him because my dh would be very upset, and mil had brain cancer and would perhaps not word it in the best way.
Everyone is different and so is difficult to advise on your child, but with my son I said something along the lines of "you know nan is very ill don't you? (ds: Yes, she's in hospital but she'll get better). "I'm sorry, and this is very sad, but Nan is not going to get better. (ds: what do you mean?) "The dr's and nurses are taking good care of nan and making sure she's comfortable and not in pain but Nan is going to die.
Lots of tears and cuddles and why questions. This sounds awful, but we took him out to buy a computer game and it cheered him up and kept him busy afterwards. The next time he saw her, I was surprised that he didn't cry. He acted like he normally would.
When she died I ensured I never mentioned "going to a better place", but we did talk about what we think heaven might be and and the people/pets she would be with. This comforted him. He's talked about death a lot since, but he's been resilient and talks happily about his Nan.
I hope things are comfortable for her and you get the care you need. It's such a difficult, exhausting and emotional time. X