K.
First off, I am soooo sorry for your loss.
I miscarried myself and know firsthand the wide range of emotions that come. my heart goes out to you. Please reach out to groups like Kindermourn if you want to get some support. Some may act like it's not a big deal since it was first trimester or because you have two already, but you need to grieve and mourn like you need to. Big cyber hugs going out to you from me.
On the question, I would avoid the whole death discussion unless this has already been a topic discussed with your 5 yr old for another death. You could simply say that mommy and daddy thought we were going to have a baby but a)God had other plans for us at this time, b)we were mistaken, but maybe someday we will have one, and/or c)this baby wasn't ready to come here and join us yet, so we have to wait a little longer.
The thought of a baby dying is simply too intense for some children at that age. They may start worrying about other women that are pregnant, they may be stressed if you get pregnant again, and they may think that God takes babies away from people. Just be very careful and know that the words are only part of what they hear--the imagery is what stays in their mind's eye....and, is what they will repeat to others.
So, do you want the 5 yr old sayingt to people, 'My mommy's baby died', or 'my mommy thought she was having a baby, but God had another plan, so we have to wait a little longer'.
Just some things to think about. Again, my heart and hugs go out to you at this sad time. I hope you have all your dreams for motherhood come true.