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Are you serious? I could reason right and wrong and use logical thought long before 10 years old. I can remember when I was 5 and I figured out that animals had feeling like I did. I learned what death was by age 7 having been exposed to the dog I'd known all my life fall down and do everything except die in front of my eyes. We took her to the vet to be put down that day cause she was like 14 years old or something.
If you are a blank page by ten years old, your parents seriously fucked up. If you cannot remember you childhood, or it took you a decade more to realize that what you did as a kid was fucked up, I weep for you. I cannot imagine what it must be like to go through life as a kid with no understanding of what the fuck you are doing. It is beyond my ability to fathom what your childhood must have been like to have caused that.
That being said, I do not advocate jail. This 10 year old girl needs counseling not because she is a blank robot slate, but because she likely has other issues. It is unlikely she meant to kill her brother, HOWEVER, a good psychologist can find out if the girl is just ignorant of her actions, incapable of understanding due perhaps to a lower intelligence or maybe some other reason, or an emotionally detached sociopath.
Either way, when I hear people say that children don't know any better, it disgusts me. It is true that sometimes they do not understand things the way adults do, but what you are suggesting (that kids don't understand anything at that age) is not only ignorant but dangerous and just plain wrong.