Are you implying that kids don't know killing people is wrong?
Yes. More or less. It's a little more complicated and I won't for an instant attempt to claim that I understand all the nuances. But basically children are mentally developing. As someone else pointed out, children (by and large) cannot be expected to understand the consequences of their actions as much as an adult.
You're arguing for special treatment of certain crimes. Eg, its OK to try to distinguish a child as a child for theft, but for a crime you find morally repugnant, such as murder, you want the child to be tried as an adult. This is an appeal to emotion.
If you read some of the information that has emerged about this story it does seem that the girls had a somewhat unusual grasp on reality.