Kids sneak out with all kinds of things. They also act differently when they get together in "packs" and no adult is around.
This whole thing is just so sad. A 12 year old kid is dead and will never have the opportunity to to grow up and maybe mature into a decent adult. A rookie cop is probably done as a police officer and depending on publicity may go to prison.
It's an environment where even the child should know it's not wise to walk around with something that looks like a real gun.