How to explain death to small children?

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My aunt has been dying for a while now. She has cancer. She was told about a year ago that it was not curable and that it would kill her. Well it has finally come down to the end. I just got a call from my cousin and she said it will be soon. They live just a couple of hours away so we have visited often over hte years. My children know her well and love her. They know she has been sick. She lost a lot of weight and all her hair. But now how do I tell a five year old and a two year old about death? How do I tell them without scaring them or upsetting them?