Good plan to speak to the teacher, I’m sure the school will be supportive . Sadly many school children lose a grandparent and the teachers know what to do.
You are right that he will struggle with the permanence of death ( don’t we all ? ). They are very big on magical thinking at his age , so he may well tell the teacher that’s she’s getting better. Or even after she has died, that she was on holiday in Spain and is coming back on Saturday.
So it’s helpful if the school know the facts so they can support him and you.
The magical thinking means that you will probably have to repeat the story over and over again for the next few months and years. Our children used to ask “ what happened to M Again?” at some random time, often when I was driving.
You get used to saying “ Gran was very ill, she had a disease called cancer in her lungs. The doctors tried really hard to make her better but they couldn’t and she died”.
Every time you say it, it becomes easier for them and you.
What BonApp said about young children asking all the details was exactly our experience too. I have to admit I smiled at having to act a dead person. Ours were very interested in the furnace etc. As adults we are used to skipping over these details and not thinking about it too much. But it seems to be what children want to know and it’s good that they can ask you and hopefully not their father or grandfather .
I suspect that at 17 your DD willl be more like an adult and find her little brothers questions distasteful or gruesome.
However I think it’s a good plan to take your DD to see your MIL tonight. At her age she will be much more scared than DS, as she is old enough to imagine all sorts of horrible things. So she will be very sad but relieved to see her step gran so peaceful and not in pain.
You need to prepare yourself because she may well be much more uspet that DS at this stage. Or she might cope fine tonight but be in floods of tears at the weekend.
Re a drip - I don’t think they do this if she’s on a palliative care pathway, although they will do mouthcare to keep her comfortable . Maybe you could ask the nurse in charge when you visit tonight ?