I have wanted to weigh in on this subject, but again, it is such an emotional subject, I haven't done it because I'm afraid it will cause conflict in the thread, like if I choose the wrong words to explain it. People are passionate about this event.
But I would say that with the little one, she's probably not looking for much info. Something very simple would be a good answer. Simple and to the point, with an eye to reassuring her. With your son, you could go more into the subtleties of the situation.
I don't have two kids, but I suspect that if I did, I'd enlist the older one's help in keeping things simple for the younger one, and keeping her from being too scared. In other words I might say "I've given you these details because you're old enough, but your little sister could get frightened by too much information. So if you want to talk about this in depth, you can always come to me and I will answer your questions as best I can." Like I said I don't have two kids, and I've already forgotten just what he could handle at age 4.5, but my son is 8 right now and if he had a younger friend and they were both asking about this, I would probably enlist his help in not overloading the younger one with info.
Just some random thoughts, but I chimed in because I saw how few people were answering you, It's a good question.