I'm 9 weeks pregnant after 4.5 years of TTC. DD is 6 and I think the news will be a massive shock for her, but I also think she'll be very excited.
I'm thinking of telling people after the 12 week scan as I don't think I'll be able to hide it for longer - I'm a bit bloated already and was showing with DD by 16 weeks. I guess I will have to tell DD then too as I don't want to her hear it 2nd hand.
Another reason for wanting to tell her asap is I've had terrible morning sickness. I told DD I've got a 'poorly tummy' (this was possibly a mistake!) but this has gone on for several weeks now, and she keeps asking me if I'm going to get better with an anxious look on her little face. I think it's worrying her :(
The main reason I'm against telling her until much later is in case anything goes wrong. It is unlikely I'll ever get pregnant again if I lost this baby as we had 3 IVF attempts to conceive. I don't want to raise DD's hopes of a sibling only to have to tell her it's not going to happen after all :(
As you can see I'm really confused what to do for the best 
Any advice from anyone re: telling an older child would be appreciated :)