My mom died of Malignant Melanoma when I was 3 years old. My 5 year old sister and I were told that our Mom would be going far away and could not come back, and would miss us very much, but that she also loved us very much.
This satisfied our little hearts and minds until we were old enough to understand what death is, and will give you time to heal and prepare for explaning more when they are older and understand more.
Hope this helps. God Bless you and your family.
A note to the Vegan, I had a friend who has been a strict vegan nearly her whole life, ate organic foods, etc, she still developed colon cancer.