As Liebs doesn't do well with change, we've already told him we plan to ttc soon. Not in so many words, of course! But changes are already happening and he loves the fact it's a secret for us 3. We will tell him about the pregnancy shortly after we know, as more changes will occur immediately (ie I will refuse to carry a 7yo around). He is already prepared for that change.

Depends on the child, but I say tell you children before extended family or work. It's only fair and does impact them more.