I was due Nov 12. I actually found out on my 2 year old daughter's birthday that I was expecting. My 4 year old guessed ~ we didn't have to tell her. She was so excited to have another baby on the way. I tried not to tell her or others given my high number of previous miscarriages but I was showing so quickly that people guessed as soon as they saw me (I was in maternity clothing by my 5th week). At 10 weeks I lost the baby. It was miscarriage #6 for me. We had seen a heartbeat and been told everything looked great shortly before. I actually told people the day before it happened that I was pregnant as our Easter surprise.
My daughter is having a difficult time dealing with the loss (we have experienced many deaths in our family, so she understands the concepts of death). Even though it is difficult for her, I am somewhat glad she understands. She isn't scared when she sees me cry. She and I have been able to keep an open dialogue and share our feelings.
In a situation like this I can understand the difficult choice of to tell or not to tell. My husband and I debated it frequently prior to conceiving but didn't need to debate long after our + result because she guessed so quickly! Sometimes kids do take the guesswork out of the situation.
Each child is so very different. Your daughter may benefit from celebrating with you early on or she may benefit from you waiting to tell her the news. I would just watch her and see how it goes. If she is very observant and patient telling her sooner may not be a problem. If she is more oblivious to the changes happening inside of you and is sensitive about death, I'd wait until the 18 week u/s.
I would suggest though if she asks you why your belly is getting bigger that you do tell her. Kids sense when you lie to them or withhold information from them.
I hope that all goes well for you and that you do not go through the pain that I have recently gone through.