S.E.
I'm so sorry for your loss - I know how upsetting it can be, and having to comfort your other children just adds to the frustration.
First, stop and think about their level of understanding when it comes to how babies are grown, and go from there.
Your 8 YO may have a clue that the whole conception/pregnancy process is intricate and more complex than just "a baby growing in your tummy," and if she does, take time alone with her to explain better what has happened. You don't have to go into *everything* about how babies are made, but just a little more detail than what you're giving. Your 5 YO will need less explanation, and will probably be content with fewer details. And, don't forget to reassure them that Mommy is going to be fine, too, and you're not the only family this has ever happened to - sometimes things just happen and we have to deal with it and move on.
Also, you might start looking at some books. It may be the right time to let them see things like "A Child Is Born" - which has FABULOUS photos. Being a doula, my 8 YO has been fascinated with all of my books as long as she could pick them up - the pictures were interesting to her, even if she didn't truly understand what she's seeing. She's not afraid to ask questions and she's not afraid of the process although she does grasp that it is complex. Taking the mystery and fear out of it can help them (and, in turn, help YOU).