book recs for a gender-dissatisfied child??

So, one of the Megatherium's social set, with whom she has been only intermittently getting along for several years now, has for some time been entirely dissatisfied with being a girl, but tells her mother she doesn't want to be a boy either, but she doesn't want to talk about it with her. The mother is a super-supportive be-yourself screw-gender-roles type who wants to help out, the kid is a loud cranky individualist who's partway between tomboy & she's not sure what, and the only age-appropriate gender-nonconforming fiction I know of is very pro-girly. Does anyone know of anything mid-grade or below that addresses this set of issues from a "but I don't like 'girl stuff'" perspective?