I cannot express just how both personally and legally dangerous this is. When trying setup or entrap someone as a citizen without being under any legal authority to do so, makes you open to being charged with the attempted version of the crime you are imitating, whether or not your intent was to carry out the actual crime. You also can be charged with various statutes mostly doing with impacting official duties of LE. I cannot stress just how much trouble this can cause. You may ask what Police or DA would prosecute such a crime? The ones who do not want to be embarrassed by their lack of enforcement for one. Additionally any "evidence" you possible collect could be considered entrapment and may not be admissible in court, especially in federal court. The person you are trying to entrap could then sue you for damages if they are not convicted.
If your really want to help, contact the the National Center for Missing and Exploited Children (NCMEC) at
Home. This organization has a cybertip line, a 24-hour hotline for missing children at 1-800-THE-LOST, and works with FBI and other law enforcement organization. They are always looking for volunteers to man phone lines as well as many other functions.