Tell them as they need to know at each progression of the Cancer.
Don't make a huge unnecessary announcement.
Let them see that Grandad is upset......he is.
"Why is Grandad upset"?
......and off you go......
Just tell the truth a bit at a time.
No need at the moment to predict the future for them, discussions of death etc.
Outline what's happening at each stage.
Kids can see through sugar coating and lies.
Cancer / Death is uncomfortable. They will cry.
Lies from parents will cause more problems.They will be angry and cry more.
I see things a bit Black & White too.