First off I want to thank everyone for their comments, encouragement and advice on these forums, I really feel out of my element with this situation and you guys have been my foundation, especially Rhodes68 and Tonith.
I managed to sit down and have a talk with my son, I asked him about the girl, and the doll. I told him he wasn't doing anything wrong, and neither was she but that she might be in need of help. He told me he has seen her in dreams, which she came to him looking for someone to play with. He says that she asked him to take care of her doll for her and then when he woke up the doll was sitting on his dresser. He said he didn't tell me where he got it before because he thought I would be mad.
I made sure he understood I believed him and was not mad, I asked a bit more about the girl but he doesn't know anything else. I asked him if he still sees her in his dreams and he told me sometimes, I told him he should ask her name, and tell her she didn't have to hide from me if she was around, that I wasn't mad at her.
I have also gotten in touch with another psychic who is a functional medium, he said he would come and have a look. I'm not sure if I want him to come while Cam is here, I'm not sure what I would tell Cam about who he is. I don't know if he is aware the girl is a ghost but if he isn't I don't want to scare him; I especially don't want to frighten him with the "brute" presence if he isn't already aware of it.
I wrote the first half of this post before when the site was not taking any new submissions, and since that time there have been further developments.
I woke up the night before last to hear Cam screaming for me. I ran to him and found him sitting down backed up against the wall at the top of the stairs. He was so scared he had peed himself; he said that he saw her fall down the stairs and then she didn't move. I suggested we go get him cleaned up and he started crying, he said he was afraid he would fall. I offered to hold his hand and help him, but he just dissolved into crying. I went downstairs got a bucket of water and a cloth for him to clean himself up.
I assured him it was just a dream, he wasn't going to fall down the stairs, but he still seemed so scared. We got him some clean PJ's and I sat with him for a bit until he fell asleep. I noticed the doll was back again, so I asked him if he got to talk to her again. He shook his head, said she walked out into the hall and he followed her, and that's when he saw her at the bottom of the stairs, then he woke up. I'm wondering is this how she died. And what about the girl at the bottom of the stairs and the stomping coming up the stairs, I don't know if this is coincidence.