I'm running in circles on this question. When I was pregnant with DD2, DD1 had already been telling us for about 2 months that her sister was coming when we found out we were pregnant. I didn't tell her. She told me. I wasn't at all concerned about her spilling the news either. We told our families when we went home for Christmas at 6.5-7 weeks, and went full public pretty much immediately after that. With DD1 we told families as soon as we knew, so at about 4 weeks. We found out during our Christmas visit.
Now DD1 is 5. It will be announcing to her that we are pregnant. She's also a chatterbox without much of a filter. Who knows what she'll say to who once we tell her. My kids are both very familiar with and enthusiastic about the idea that we hope and plan to have more babies. I'm not expecting the news to be shocking to them.
We don't plan to travel to see family until Feb or March, and I'd love to tell them in person, but that would be waiting quite some time to tell. Right now I'm thinking waiting until 6-8 weeks sounds good. 9-12 sounds too long. Besides, I don't expect to be able to easily hide it much past 8 weeks, and I don't want to tell our local friends and keep it a secret from our families. I'm assuming once we tell the kids, we'll pretty much have to be comfortable with telling our local friends that we see all the time.
Also, I'm really hoping to do a weekend couple trip with DH in early to mid second trimester. We've never spent a night away from either of our kids, and I'd really like us to get a bit of time away together before we're adding in a whole new tiny baby to the family. If we do that, I'd be asking my parents to keep the kids, and I wouldn't feel right trying to make the plans with them without divulging the pregnancy... So, I guess we'll tell over the phone at 6-8 weeks