Tough question. When I was growing up my Mom sat me and my sister down when we were close to becoming teenagers and told us that if she ever caught us with drugs, using drugs etc', she'd call the cops herself. My Mom was one of few words and when she said something she ment it. We knew she would do it to and that stuck with both of us. It also had a lot to do with our upbringing.
When my wife and I have kids, I will try to do the same type of upbringing for them. I'll be teaching them about firearms at an early age and I'll have the same conversation about drugs with them that my Mom had with me. If I find them with a handgun, whether it's mine or not, I will first ask them why. If the answer is not to my liking, I will involve law enforcement. The consequences of not doing so could be life altering for the rest of my family and countless other families as well. Not sure I could live with myself if one of my kids got another handgun and did something with it after I knew about the first one.