"I would like to know if you have heard of older children (6-8 years) scratching infants for no reason? There is this girl in my neighborhood who came over last week and left deep scratches on my daughter's arms and legs. I don't know what steps I should take in "disciplining" the girl, but I am very very very upset and confused. Can you please provide so insight on what possibly could have prompted her to do so and what should my husband and I do as angry parents?"
Wow. I think I'd be completely shocked and livid if that happened to my baby. A child that age should absolutely know better than to hurt a baby.
I'm not a development or psychological expert, so there may be something going on here that I'm not even considering. But the first thing that occurs to me is that the little girl may be jealous of the baby and may be trying to hurt her. If the girl spent time with you and your husband in the past and felt like you had a special interest in her, then she may be very jealous of the baby for taking your time and affection. She should know better than to hurt the baby, but the jealousy would explain why she did it.
The other thing that occurs to me is something that I hope isn't the case, which is that the girl herself might be a victim of physical abuse. Kids who grow up being hurt don't know that it's not normal until they're older (and some of them never realize it isn't normal, which is why they pass it on to their own kids). So if she gets hit or scratched, she may think it's a normal way to interact with a baby.
In this situation I think I'd take photos of the scratches to make sure I had a record of them. Then I'd call the girl's parents and discuss with them what happened and make sure they know how upset you are, but in a non-confrontational way. Let them discipline the girl. (I think an exception to this would be if you think the girl herself may be a victim of abuse. In that case, I'd call a social worker to get ideas about how best to proceed with this. If the girl is being abused by her family you can get them all some help, which will help your situation as well.) In the meantime, she shouldn't come over to see the baby until you're sure she's not going to hurt the baby again, which may mean she can't come over for months or years.
I'm very sorry this happened to your daughter, and I hope you can come to some kind of resolution with the girl and her family.