This is a question I ponder often. I don't think there is a right answer. My daughters are 4 and my oldest will be 6 in two months. They were 1 and 3 when I was diagnosed, so they have no recollection of having a mother without cancer. We have always chosen to be honest with our children, but only give out the information as they ask for it or if my husband and I deem it something they need to know. They both understand, particularly my oldest, that I have cancer, and that cancer is a kind of sickness. She also understands that sometimes people get so sick or so hurt, there is no way to "fix" them, and they die. We haven't connected the dots for them, as far as telling them that eventually I will become so sick that I cannot be fixed, but my hope is that we have been clear enough about the life cycle, that these discussions will unfold fairly naturally. When I get to the point that I have exhausted all my treatment options, and my demise is imminent, we will be honest with them about what is going to happen. One thing I was told to remember, by a psychologist, is to not underestimate the intelligence and perception of young children. They absorb and understand much of what is going on around them, even if they don't verbalize it. Much of what we will tell them, while certainly distressing, will not be as big of a shock as we might think. Additionally, and this is speaking from the perspective of losing a parent at a young age (my biological father died when I was 4), I remember clearly being told of my father's death. There were no tears or hysteria. The permanence of death is difficult to comprehend at such a tender age, and the understanding and grief is much more of a process than with an adult. I worry far less about my children's resilience, than that of my husband or particularly my mother. My death will not destroy my children; it will forever be a fundamental part of who they are but they will be OK.
I think however you choose to handle this with your daughter, it's not going to be the "wrong" way. I do think honesty is best, just present it in a developmentally appropriate way. Social workers, psychologists and pediatricians are great sources for guidance on how to handle your particular situation.
stg IV 4/2013 @34 - liver, ovary/peritoneum
Lots of chemo, surgery and good luck - still doing well 03/2016...