I'm so sorry to hear this.
We're not religious so have had to give fairly realistic explanations of what happens, rather than 'you'll see her in heaven etc...'.
2 books that are really nice are:
Badgers Parting Gifts, is a beautiful picture book, about an old badger who spends some last nice time with his friends, and dies, and how whilst everyone mourns over the winter, and that's ok, they slowly start to remember all the wonderful things badger did for them, and teach each other the skills he taught them, and so badger lives on in your memory.
Paper Dolls (Julia Donaldson) is less death-specific, but again shows that people who die live on in your memory.
They're for smaller children, but I don't think that's a bad thing when older children need reassurance and to have things explained simply.
We explained that doctors can't mend all problems, and sometimes when people are very very ill, their bodies can't carry on any more. We explain death (please do not describe it as going to sleep, as that can terrify children), and we even had a little field trip to see a recently dead squirrel, as the concept of the life going from someone is really hard to describe. We then talk about nothing being able to hurt them any more, they now live on in our memories, and though their genes, in us. We are carrying their stories around inside our cells. We then discussed burial and cremation, and described how energy is never lost in the universe, and (sorry about saying this, but this did help the DC) so once their body begins to return to the earth, then this energy goes into creating new life - trees, flowers etc. The children are pretty cool about the whole thing, and often wander in graveyards commenting on how all the wildflowers around are life continuing from the people buried in the graveyard. It's a scientifically sound explanation, and surprisingly comforting.
Still got questions over time such as, what if they want to come out of the box?' but we just reiterated all the above. You can describe it as their soul leaves the body and lives on in our memories.