This is clearly a negligent babysitter issue. Not everything that happens to kids is the fault of parents. This is why I never hired babysitters when my kids were little and the parents of the kids they played with knew that I wouldn't hold them responsible if they got hurt because my kids were taught to take responsibility for their own bad behavior and to respect the homes of their friends. Bumps and bruises are a part of childhood but most parents teach their kids that guns are not toys. Most kids are smart enough to tell a real gun from a toy although some toy guns like pellet guns, bb guns, paint ball guns, etc should be kept out of the hands of very small children. I'm sure that most people have at least one  painful memory from childhood but most of us are alive to tell it. Being a parent and being a kid is tough.