When I was twelve the last thing on my mind was what type of panties I should be wearing. Seriously! I'm 19. It wasn't THAT long ago, was it?
The ONLY reason that I could see such a young girl wanting/needing (it's even pushing the limit to call it 'needing' for this reason) is if she's in dance or something where a pantyline would draw attention. But even then, if she's onstage, not too many people will notice.
I agree with raeanna. Bikini, high cut, low cut, no thongs. She doesn't need the infections, she doesn't need to sexualize herself.
Send the girl my way, we'll have a chat.