Wow... It's not easy to get attachment disorder, what was done to her when she was little? I have worked with children that young (a few, not many) who have similar diagnosis and my thoughts about this envision a little girl who enjoyed what she did. It is totally my opinion and a guess, but I would be willing to bet that she is a young sociopath, lacking emotional connections to others, who did this for pleasure and curiosity. I hope her status as the youngest in so many years gets her special attention from mental health specialists in the field, cause I think she is very dangerous.
As far as babysitting at ten, I did and was very good at it. SOME kids that age can... I think the neglect is not in the age but rather the mental disorders of that child. Especially since she harmed another baby before! I just hope the baby's suffering was very short. And the ten year olds mothers suffering is very long.