This makes me physically ill. When my son was little, a car of teenage boys ran up onto the curb to run over his little dog while he was playing with it in my ex-husbands yard. I swear, if I had been there I would have chased them down, pulled them from the car and beaten them to death. (I'm sorry Moderators, but this kind of thing makes me crazy).
First, I'm so sorry that this happened to you. Second, 6 and 7 year old boys shouldn't be running around together without adult supervision. Find where one or some of them live. Call Child Protective Services or what ever its called in you state. The boys are too young for the juvenile system to do anything about them, but thier parents letting them run loose, unsupervised killing other peoples' pets will be taken seriously. The parents will be hauled into court and be in a system that makes them accountable for how they are taking care of their children by completing classes and programs.
I will never understand such cruelty in a human of any age to a little animal. I am so sorry.