Well, its up to you... as we women know, the first 3 months are the crucial times... when I was pregnant a 2nd time... I was only about 6 weeks pregnant... we hadn't told our daughter yet... and luckily, because I had a miscarriage.
With my 3rd pregnancy, with my now son... we told our daughter at about 2 months... and she was very happy.
I took her to all my pre-natal visits and my OB/GYN actually encourages it and doesn't mind... my Doctor even taught my girl how to put the doppler gadget on my tummy to hear the heartbeat, and my daughter would do it at each visit. And my Doctor and I would just instruct my daughter to stay near my head or upper body area... if anything "private" had to be examined. My daughter was fine about it and she LOVED going to the Doctor with me.
I did not of course, take her to my ultrasounds in the 2nd trimester, because it was for my amniocentesis as well. But for regular monthly pre-natal visits, I took my daughter. She found it so fascinating and she was proud to be with me to check her brother at the Doctor.
But its a personal decision.
Each month as well, I took a photo of my growing tummy WITH my daughter right next to my tummy. She LOVED doing this... and we often took pictures together with my tummy... and she would even take pictures of me herself... of her pregnant Mommy. She was about 3 years old at this time, and she could use our digital camera. It was a nice photo "journal" we had compiled. Just the 2 of us... and my tummy with her brother in it.
At 6 years old, a child can understand a lot. Also about "privacy." My daughter, understood what "privacy" was and that we don't just go around telling people everything that is personal in our family. You just explain to them. But if she can't help it, then you need to understand and just be prepared for your "private" news being broadcast.
OR, wait to tell her until your tummy growth starts to show.
All the best,
Susan