First off, anyone who says they will perform an exorcism but only for a certain amount of money isn't someone to trust. Finding out about the little girl is a simple matter of checking town records. Don't just check into the house specifically but the property. As for why it started recently or what it might be, find a reputable medium & that may give you some more leads. Iron, salt, Dragon's Blood oil (its an herb, readily available at any new age store), smudging every room of the house in particular her room & her son's. I wouldn't encourage her to try communicating with it herself through anything like a ouija board since that is likely to escalate things.
Depending on where she lives she might wanna worry about ley lines.
You know how they can be sensitive at that age, specially with moving and all....