I cannot even imagine how the mother must be feeling.
IMO, I think the adult sitter should be charged with manslaughter. Like most of you have said, she was the adult in charge and should have been keeping a closer eye on the baby. The 10 year old I would say knew she was doing wrong to the baby, but probably didn't realise she was going to cause the baby's death. Therefore, manslaughter is too much and the child should have some councelling!