Speak as honestly and age appropriate as you can. You may need to talk to them separately.
My eldest son was just before his 16th birthday when diagnosed with a brain tumour. His brother only 13.
It all happened very very quickly, youngest son went to school and by 11 am I was in hospital with my eldest where he stayed for 6 months.
We answered every question with as much honesty as we could. We were led by both children as their needs were different.
Youngest son was very frightened of seeing his brother in hospital. We set up Skype and they kept their bond talking every day (if eldest son was able to). We kept his brothers visits to a minimum (a lot of times he couldn't visit as most school aged children were not allowed on the ward due to Noroviris risk).
Don't be frightened of showing tears or anger. Give them the words that they can use.
I told my sons they could swear all they liked at cancer. F word was acceptable when talking about it. It gave us all a word that expressed our anger, fear, tears.
My youngest son wanted to know the reality of what was happening. My eldest (who was the one with cancer) wanted some reality but to always talk about hope and the future (even when he knew he was not going to survive). Each of your children will have different needs and you will learn them as you go along.
Give them space to express their fears, their anger, their tears.
My youngest wanted to go to school as normal, it was his escape. He told very few of his friends what was going on. It was his way of dealing with it at that time. We offered him counselling but it was 4 years after his brothers death that he felt ready to accept counselling help.
It's a difficult frightening time for an adult and more so as a child.
If the person affected is very much older it can be accepted as part of the circle of life. If the person affected is much younger then it is very difficult to express the fairness of any of it.