Because the story is your general Sword of Shannara thing (I have *no problem* with treading familiar territory: nobody can tell the same story I'm going to), but my vague impulse is to Mulan it: last chapter, our 'hero' goes home, takes her hair down, and kicks back. None of the story is about worrying about being Found Out, so 'she' is 'he' all the way through.
I wonder if you can even *do* that in fiction, or if it's telling too much of a lie. This is how the original Mulan poem ends, after Mulan has returned home and put her girl clothes back on:
I step out to see my comrades-in-arms, They are all surprised and astounded:
'We travelled twelve years together, Yet didn't realise Mulan was a lady!'"
The male rabbit is swifter of foot, The eyes of the female are somewhat smaller.
But when the two rabbits run side by side, How can you tell the female from the male?
I kind of love that.
The society would apparently have to be one where women/girls didn't typically go off to adventure, because otherwise there's no reason to Mulan it, except for my general belief that books aimed at little boys in particular ought to do their best to change gender expectations. The other thought is just to make the hero a girl from the start, but I /am/ writing these middle grade books mostly for little boys (my nephews have NO IDEA, NO IDEA how amazing this is), so I'm more inclined to try turning things on their ear.
Y'know, two years down the road when I get a chance to do this idea, anyway. :)
eta: over on Twitter, when this discussion was put to them, with the follow-up of "It's a trilogy. Reveal at the end of book 1 & risk losing audience or keep it up for 3 books?", Fred responded with, "Rite of passage - 1st, conceal from reader; 2nd, conceal from world, conspire with reader; 3rd, confront world, ally with reader," which I think is a *perfect* approach. Hah! Yay!