I think you have to go with your instincts on this one - I can tell you what I did, but of course it might not be the right approach for you.
I became pregnant when my SD was 14 and my SS was just turning 12. My partner and I were not living together then; he was a LP so lived with the kids fulltime. My relationship with the kids back then - SS and I were very close, lots of talks and cuddles. SD and I, not so much, lots of tension there.
My biggest issue with SD has always been that she tries to control things, and DP let her get away with it far far too much. So that affected how we handled the pregnancy - we didn't want to encourage her to feel like a third parent or like she had too much say over our decisions. She did say things duringthe pregnancy like "well, you obviously can't have any more; you are too old"!
Anyway - in our case - DP told the kids himself, without me present. We thought that would allow them to express their feelings about it more easily. I went over to their house that evening. When SS went to walk the dog, I went with him, and we talked about it, which was nice. I was pretty worried (and still am!) about it screwing things up between SS and I - we'd been close, and I didn't want him to feel rejected when I had so much less time or attention for him.
We told them at about 4 months, I guess. Like I said, there were some risks to the pregnancy, and I wanted to be sure it was 'sticking'. I also had plans to see my parents around that time, and I wanted to tell them in person; I was a bit worried about the kids letting something slip before we announced it at my work, or it getting around before I could tell my family myself.
We didn't take the kids to the scans or anything. (I had some problems, so we were getting scans every few weeks to check on the baby.) Partly this was because DP didn't want them to miss any school, but it was also my preference. I wanted to share those moments with DP alone. Also - SD was insistent that the baby "had to" be a boy - I didn't want her to find out it wasn't and be a pain in the ass for the rest of the pregnancy!
We told the kids what names we were considering, but never suggested that they had any input into choosing. As far as I was concerned, that was up to us. When they have kids, they can choose names then! (Actually, long story, but SD pulled a stunt about that - telling someone that we had decided to name the baby after that person - before we had made any decision - becasue it is what SD wanted - awkward!!! I'm still angry about it!)
The kids were actually really happy about having a new sibling. For SS, I think it was reassuring. For SD, I think it was preferable to having me around - if she could have had the baby but not me, she would have! In the long run, though, the baby became the force behind a lot of positive changes, and it has made us all feel more connected to one another, I think. The kids love her to bits.
Anyway - long post, sorry - in my case, I chose to keep the kids involved in conversations about the pregnancy, but not to give them an active role in decisions. That was right for us.