I don't have kids of my own, but I can relate to your story a little. My step brother has a son who is now 10, he's seem me as his "uncle" since age 4. He mimicks every thing I do, tries to dress like me, talk like me, pretty much is my little mini me. He's a very bright, open minded little boy. He's grown up in the bay area, and both of his parents have friends in the glbt community. Told me at age 6 it's "ok for boy to love another boy", because he was taught not to judge, anyone.
Well up until a few months ago, he knew nothing of me being trans. He knew me as his uncle J. Unfortunately his mother and father started having issues, decided to seperate, a whole nother story in itself. And his mother decided to be a (rhmes with witch) and tell my nephew without me present, all about me being trans. She had brought it up when he was about 8, saying she "knew he'd understand, and he needs to know eventually". Ok, that's fine, but I'd rather tell him myself, and didn't think he was really old enough to understand. Well, she told him while they were on vacation. I was living at their house at the time, he came back, the first day he was kinda quiet around me. But the second day he was back to being the little annoyin turd he was before, didn't really ask me any ?'s. I know he doesn't understand fully, but one day I will explain it all to him. I know he still sees me as his uncle J.
I think your kids already have very open minds. Even if you're a "mommy" from 8-6, I assume they know you're in a relationship with another woman? I definitely think they'd benefit greatly from knowing NOW, rather than later on. They're still young, and learning, and would be able to get used to you changing. I agree with Andrew, kids are so much more accepting than adults. Whatever you decide to do, I wish you the best of luck.