If there was no "check-in," then there was no pre-registration for any of the individual workshops. So any 12, 13, or 14 year old could walk in to a workshop where explicit homosexual sex acts are being discussed.
Some parents, believeing this was a "safe-schools" conference, may have given permission for their children to attend the conference in general. It's unlikely they knew that this sexually explicit workshop would be included. Of course, since no registration or parental permission was required for individual workshops, a kid could always tell his parents, "oh no, I didn't attend THAT workshop!"