Upload & Sell: On
Is she trying to solicit the agent to represent her, or has she already signed with the agent?
Have you spoken with the agent or reviewed the agency's portfolio / models to see what styling they lean toward for some clarity of direction?
While she may be your client, but there's an old adage about striving to impress the boss's boss. Can you review the agent's requirements / guidelines and/or solicit/incorporate the agent into the creative process as to what he would like to have? That isn't to say you don't have creative license ... but sometimes it is easiest to answer a question that you already know the answer to, rather than guess.
Without any input from the agent ... I'm going with clean and demographic lifestyle images ... cell phone, PC, mall shopping as my muse ... what else do 12 year old girls do? What will her (future) images (broad concept) be used for that she will be cast for ... promoting marketing to that demographic or portraying a personality. You can also flirt with innocent love and boys as that is not the same as "sexy" as girls (some) are starting to turn toward WANTING to be more grown up.
While SHE might BE a 12 yr. old ... she could be called upon to portray an older (or younger) girl. Show the range of character that she could be reasonably cast as. A 20yr old "hottie", not so much. A fifteen year old girlfriend, more plausible. Does she "look" 12, or does she look 10 or 15? How is the agent going to want to use her ... which then becomes how will the agent's client want to use her. Therein lies your answers ... i.e. what will meet the needs of the boss's boss.
Also, is this going to have international flair where you might want to incorporate Japaneses or European styling ... or will it be "Down Under" flavoring only. Mostly more questions here ... but at the end of the day, I think your answers have to address them (or similar). This is the strength I play to, and then build some out of the box/set yourself apart stuff to go with it.
Gotta know your target audience ... I start with the agent/agency. Even if he wants "fresh & different" ... he should be able to at least identify his target markets. There is quite a lot of range between "child" and "sexy" ... let them steer you, rather than guessing ... but without further direction, I'm going for lifestyle and/or character @ hobbies/interests of the demographic.
HTH ... GL