Difficult question. This is just my opinion & experience. I am not an attorney, nor am I giving legal advice.
I was at one seminar where a producer was talking about his experience running afoul of all kinds of legal issues in Utah - and rescheduling to reshoot some scenes in a neighbouring state, where there were no issues at all. Problematic, but it was interesting to see how accomodations need to be made to keep inline with local laws, theoretically based on popular local mores.
There's also been the more recent phenomena of "sexting", where minors have sent topless (or even just underwear) images of themselves to boy/girlfriends, etc, and ended up with various felony kiddie-porn charges leveled at them. (PA had several high profile incidences of this, in the past few years)
Without an attorney having vetted your particular situation, I wouldn't even think about it. Not even a bra situation. I'd even shy away from having an 18+ actor playing the part of a topless minor, without legal counsel.
Also, keep in mind how this might affect your potential distribution.
Good luck.