The minimum required to stop them and the maximum that their victims permit me to do.
If I depend on a victim's help in order to catch the perpetrator and if I use that to go beyond what the victim permits, then I would just be stealing the victims right to retribution for myself, and I would be just another person violating some kid's right.
And if I take more action against the perpetrator then what is necessary in order to prevent them from abusing more children, then I'd just be uniquely dependent on the continued existence of child abusers when furthering my goals.
As a Libertarian, the way I see it is that respecting the rights of everyone involved is absolutely essential for figuring out what kind of responses are appropriate, and what kinds of responses would make me into just another sick human being.
What about you? If a misguided child is being abused, would you ever look for ways to exploit and manipulate that child if it might let you get at the abuser?
And do you think that the possibility of torturing or obtaining retribution against abusers creates a kind of goodness that wouldn't exist in a world without abuse?