Going unreg for this one...
I'm the eldest child in a family of 2, but my mum had a miscarriage before I came along. I think she had gone about 7 months before the baby died, so it must have been so distressing mum and dad.
I remember being told from a young age about my 'sister in heaven' who had died before I was born. I think I must have been 3 or 4. Obviously I wasn't able to grasp the full grevity and seriousness and sadness of the situation, but I was able to understand the fact that I had an older sister who had died.
They didn't talk about it much (or maybe that was just when I was around), and even though they still don't mention her much, I think it may have been due to the time that it happened. In the late 70s/early 80s people didn't discuss it much, and couples weren't given time with the baby or advised to have their own burial for the baby. Thankfully things are different now, and people can discuss miscarriages without a stigma or awkwardness, which probably makes it that little bit easier for people to deal with and a topic which they can appraoch more openly with their children (and I say that without meaning to belittle the sadness and sense of loss of a miscarriage).
Obvouisly you know your children best, but I would have no problem beginning to mention your miscarriages to my children from about 3 or 4 (when they're old enough to understand death). For me, I think my mum's way of introducing me to the situation was ideal, because she didn't go into any details. She just told me about my older sister who had gone to heaven because God wanted her up there rather than down here. So it was something I just accepted and grew up with. It didn't freak me out or upset me. It's only in my adult life, from abot 25 onwards, that I really started to think about it more and more, and actually want to visit the grave. Even though it only began to really sink in then, I'm still glad my mum told me about my sister, and it was nice to know I had a sister, even if she wasn't around.
I would hate to have been told when I was older, especially in my late teens or early 20s. Life around that age can be difficult enough to handle without learning about siblings who hadn't survived.
Children are very resiliant and it's often only when they're young that they would ask you questions about what happened. I know now (I'm 32) that I wouldn't ask mum and dad any questions about what happened as I don't want to upset them as they've gone through enough and I don't want to open a healing wound again.
Obviously you and your husband know your children best, and will know the best time/way to approach the topic, but I feel the way my mum handled it with me was great, and I definitely would tell your children about their siblings who are no longer with you.
I'm so sorry for your loss, Cathy, and congratulations on the birth of your two healthy children. I know whatever you decide to do will be the best for them! Take care.