My kids were the first people I told (well, DH was there, too, but we'd been talking about how it was pretty likely for a couple of days so he wasn't surprised when I made it a statement instead of a question.) I'm a CBE and a doula, and they've all been born at home - the oldest was there for the second's birth, and the older two were there pretty much immediately after the third's birth. So they are pretty familiar with the whole idea! DS1 was really happy and excited (which was really nice to see), DS2 immediately started talking about how he wanted to be involved in the birth (cut the cord, something else, which I forget, but should ask him about again closer to the time), and DD said, "do you really have a baby in your belly?"
I think it's just a good idea to be prepared and plan to be unoffended by any reaction the kids might have, all the way from exuberant joy to strongly negative to "oh, okay, can I get back to what I was doing now?" And if your kids don't know a lot about conception/pregnancy/birth, have a plan for yourself about how you want to respond if they do ask questions - generally it's good to answer questions in this area as simply as possible, because often kids don't know enough to be curious about the whole shebang at once. If they ask more, you can tell more, but especially because sexuality is a topic that's challenging for many adults, it's easy to go overboard and get too technical too fast in our explanations!
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