I have two daughters, six and eight, and I haven't told them yet. Not because I don't want them to know, but because I wanted to reach a point at which I knew what the hell was happening before I broached it. So that is now. I am not looking forward to it, but they are bright, balanced little things with very open minds. I am sure they will be a little upset, but they have the love and support of their parents and their extended family, so they will cope.
Personally, I think there is no reason why kids should be emotionally or psychologically scarred by this, providing it is handled well. Your daughter will likely take her lead from you. If you treat this as just something that some people go through rather than being a huge, mortifying crisis, I am pretty certain she will take it the same way. The key, I think, is to be open and honest (as you appear to be already) and don't implicitly give the impression that there is anything "wrong" or "shameful" about the whole business.
It might also be an idea to talk to her about how she will tell others about it. Why not just talk to her about how she can disclose this to her friends? Agree to a name that she will call you, see if she wants to practice in role play how she can talk about this to anyone, just try and normalise it. Can you think of any transpeople that she would see as positive role models that you can talk about with her (Buck Angel is probably not a good idea...)? Can she meet any other transpeople, to let her know that you are not an isolated case?
Just think, children are more adaptable than adults. They have open minds because they don't yet know what is "normal" or "acceptable". So many more things are possible in a child's world than in an adults.
And research shows that it is the quality of the care children receive that helps them become balanced people, not how orthodox the care-giver is. You sound like a caring and considerate parent.
And finally, I know that we, as parents, seek to protect our children from all the potentially harmful things in this world, particularly if they are things that we might do. But we just can't keep them in cotton wool. If it is important (as transitioning is), there is a good argument for taking a calculated risk and going ahead but with a careful, considered approach.
That was a lot of ramble, wasn't it, but I hpoe there is something in there that might help. Good luck to you.