i have two kids.  One is a junior in high school now and the other a junior in college.  Great kids and we are close. 
They almost certainly know something is up but we all dance around the topic.  There are those subtle clues like a decent set of knockers that might have led them to think something is up.  So far they think they are a side effect of the medicine that makes me a nicer person.  it wasn't a lie per se but yea, then again it was.  Anyhow, I'm going to have "The Talk" in a few weeks when it won't mess them up with finals. 

My current plan is to take us out to a decent lunch, sit down when we get home with a bottle of tequila on the table and tell them something like, "Ok, we need to talk about something.  I'm going to do this like a bandaid so hold onto something.  Yes, I'm changing into woman but i don't like guys and Mom and I are staying married.  This is a good time to take a shot."  The rest of the discussion would be describing my life with wanting to be a woman since i was 5 or so, how i am much happier now and how this will be a slow process that takes into account their feelings.  I'm not about to show up the next day in a dress. 

Does anybody else have a tale how they did it and how it went?

I gathered my two kids (15 and 17 at the time) and told them I was transgender. I tried not to make a big deal of it. I didn't feel like I needed to alarm anyone. I asked if they had any questions. They said no. They weren't sure how it was going to affect them, and neither was I.

As long as you're honest and open with them, there's no wrong way to do it.

I was in no hurry to tell my kids as this was my second coming out.  When I first came out queer/bi in the 80s it was very challenging for my son while my daughter adapted well, loved my boyfriends and we were one big happy family in her eyes. 
When I came out trans a bit over two years ago it seemed much harder for them to understand.  Though I tried to reassure my son that, like being queer, this is not catchy and does not say anything about his masculinity.  i could have gone slower but that is not my tempo and i think the results would be the same.  I invited my daughter over and there i was in a new dress.  Within 10 minutes she was cool and ready to go out and get something to eat at a public place where the wait staff ma'amed us.  My son didn't want to hear about it and has since slammed the door on any contact.  He did offer what I suppose he thought was a put down; "My life does make more sense now, since I felt like I was being raised by two women anyway"

I think our children often have an innate sense of who we are.  When love and trust are established early they can roll with our changes better.  My son was far less trusting but still acutely aware of what a sissy fairy girl his dad was and is still.  It was OK before he went to grade school but after that I became an embarrassment for him.  The gender police demons of our culture, that we internalize, are still his undoing today and he works overtime to be the most masculine guy possible.  A redneck skin head who treats his family like dirt.

Calm simple explanations and loving support cannot make up for an already damaged relationship.  Much depends on where we are at before the genie comes out of the bottle.  Going forward I hope we are prepared to be calm, non defensive and ready for a range of emotional responses that may greatly improve over time.  I am wishing you well but recognize my 50% success rate as a caution.