S/he would have to be, really ... The White Mountains is a bit of a boys' own adventure. The only significant female character only lasts a couple of chapters before coming to a sticky end. A modern family blockbuster would need there to be a strong leading young female alongside the boys. So it's either invent a fourth character to join them, or else change one of the three.
Anyway, look on the bright side, they might cast Clemence Poesy.
