Infrared video camera and illuminator? I don't know cost, but Kodak used to take photos in a coating plant while work was in progress recording an image of the workers and film or paper using IR film.
Part of me wants to say all this is making more of a threat than really exists. The news media make it sound like every adult wants to molest every child. The reality is that while molestation is a tragic thing that does occur, it is more likely harm will come to a child by an automobile accident while being driven to the youth center.
If actual risks were the issue, we would insist upon four point harnesses and rollover cages in cars. The risk from molestaion is quite real, but has been unrealistically magnified by the media's response to it's repugnant nature.
Anyway - an IR imaging system would solve the conundrum.