I would call the local police non-emergency line and explain what I had stumbled upon and ask them what I should do. That way, especially if it was something like child pornography, they could be immediately alerted, someone could look at my computer and possibly be able to gain some information about the source and hopefully find clues that could lead to the rescue of the child if they have not yet been found.
For something like an obvious scam, it probably wouldn't be as big of an issue, they might just want you to send them the links and a detective would check it.