This is a tricky issue. How to educate the younger-than-18 audience about BDSM so they may draw informed conclusions and decisions about what we do and what they might want to do when they choose their relationship parameters.
On the one hand, a person of legal majority leaves themselves way open to civil and potential criminal legal issues if the parents of the minor find out that an adult has been talking to them about BDSM. Let's face it: BDSM has been given plenty of black eyes by the uninformed and the criminal already; we do not need more.
On the other hand, if people are not willing to educate others about BDSM and its hazards and benefits, then how are today's youths supposed to learn about real BDSM, and not just the criminal crap they see portrayed all to frequently on Law and Order and American Justice?
The simple answer is to inform the enquirer of legal minority to find the nearest Barnes and Noble or Borders and check the Psychology or Sexuality section for informative books on BDSM. Since books are legal purchases for any person of any age (except adult magazines), the minor can legally purchase the informative books and read them and become better informed about alternative sexuality.
Otherwise, all I can say is: Stay out of it. BDSM on the internet is often hidden behind "Adult Content" warnings. Even informative sites like Castlerealm have an "Adult Content" warning and they do not even have pictures on their site. Regardless of this, any BDSM information that is to be gained by minors needs to be gained from books. Trust me, you do not want to be a registered sex offender for having simply shown a minor a link to a BDSM website. You can never run away from that stigma.