sorry to hear, very sad news 
does she understand death as such?
you can't avoid her getting upset, as you yourself will be upset, so it's best to just say the simplest things and encourage her to ask questions.
I'm sure you'll find the right words, but you can also say that it is hard to explain - it's good for children to realize that we don't know everything....
if she's seen the Lion King she'll have a vague idea, I'd watch it with her anyway, it might be a good starting point for a conversation on a child's level of understanding.
now of course this is a very difficult subject even if you are religious and possibly even harder if you are not, so I'm not going to suggest anything for you to say either way.....
....but when I had a miscarriage 4 years ago we told the boys (then 7, 5, 4 and 3) that the baby was too ill so God decided to look after her in Heaven instead of letting her be born, as she would possibly be in a lot of pain and might die anyway.
we are Christians and truly believe this is what happened, the children understood and accepted our explanation.
we ended up talking a lot about where babies come from actually, but any questions they had about the operation (ERPC) or whether it hurt baby etc I answered as honestly as I could (no gory details of course).
some things I couldn't answer, so I told them I didn't know.
I don't know if this is any help, and again my sympathies to you and your family