[imported_Fluffy_Bunny]

I believe in telling kids the truth, or a simplified version of the truth as you have done. They don't need to understand the whole situation, just the basics & telling her that the little girl is 'gone' is a good example.
Children will eventually find out the truths for themselves at some time, once they watch & understand the news then they will begin to learn. There's no point going into explicit detail because you will probably mess up their head. The first time I realised the world isn't all its cracked up to be was when I was about 15 and read a book about the Vietnam War called Chickenhawk. The author went into some detail on what the VC did to captured pilots...
I also found the site 'Massacres of the Second World War' which was quite interesting.