I told DD as soon as we found out at 5 weeks ish (she's 9) a because she had to go to the doctors with a rash that I wasn't sure whether was dangerous for me being pg and also because I was gagging and retching a lot and she was a little worried that mummy was sick
When my dc were younger I waited till 20 week scan. . Too long to wait otherwise. Older ones at 12 week scan when ds 12 commented ony big boobs!! Wouldn't have dreamt of telling anyone before 12 weeks as too hard to untell a dc if it goes wrong in the early days.
We told dd (2 yr old) at around 12 weeks, just before everyone else. It was important to us that she felt involved, and we talked about "her baby" and "her new brother or sister" throughout the pregnancy. Worked well for all of us