Hi Karen,
I used to be a bookseller, so your question interested me. I know there is not much, if anything out there for someone in exactly your situation. There is, however, a relatively new children's book called 10,000 Dresses by Marcus Ewert (Amazon has it) that is about a transgender child. It shows the basic dilemma which is addressed with humor and compassion. Your library may carry it, or should! It might serve to introduce the concept of gender variance and hence be an introduction to your explanation.
I also suggest that you try contacting someone at PFLAG. Do you know about Pflag? Parents and Friends of Gays and Lesbians (and now trans as well). There may be, as you would hope, a pamphlet or something. Another group is also a group in the US specific to helping trans kids that may also have ideas. I can't remember their name right now. But I could research it if you think you'd like to contact them too.
Like Nicky says, young ones are generally accepting, that is, if you are, which, blessedly, you are.
Hugs and best wishes to you.
Ruby