You did the right thing by backing off, especially since your son was with you. Was it possible to get their license plate number? If so report them to the poaching hotline. I think the important thing in your situation is to talk to your son about what these guys were doing, and explain to him that these guys were breaking the law. Even if you could not get the license number I would report the incident(date, time, location, etc.) any way. Maybe they will increase enforcment in the area if they are made aware of this. I reported some killing of native steelhead on my home river earlier this winter and was suprised to see some enforcement the next weekend.