We were in this position, we have two children though DI. We also had counselling before we had the treatment, and through this process we decided to tell. We had also done a lot of research online, this question has been really extensively studied, and it seems that children who are told naturally as a part of their "how did I get here?" conversation and who are told young (under 7 at least) are absolutely fine about their origins. Those who are told later in life are generally very upset about being lied to by their own parents for so long (and I can see why).
I had concerns about telling my two. My Dad was against telling so early, because we are very private people, and you can't rely on a small child not to blurt the news out to all and sundry! But actually, because it wasn't told as a "sit down we have something Very Important to tell you" thing, but just as an answer to the question "how was I made in your tummy?", it hasn't been a big deal. I was actually surprised how easy it was to chat about it. I also told them about the sadness that we had in our lives when we thought that we wouldn't have children, and how happy we were to eventually have the two of them. It turns out that our children are as private as my husband, and don't want to discuss it in public. To be honest, even if they reach the stage where they do, it's at least as much their story as it is ours.
I suppose as they grow older, it will have more significance as they understand it more. We (or rather I, they always ask me!) have answered any questions as they have turned up. The younger has learned everything by osmosis by listening in to chats with the elder. But it will never be a shock to them, and they will never remember not knowing.
I know that a medical problem with sperm generation shouldn't be shameful, but I'm afraid that there still is a lot of prejudice out there. We all know jokes about "jaffas" and there is some misunderstanding correlating infertility with impotence. But my husband has decided to take this on the chin, because we feel that this isn't our decision to make. Our children deserve to know who they are, and we can't take that away from them.