I think only Tolly and Ms. Oldknowe can see Toby, Linnet, and Alexander, because they were part of the family and the only ones that live in the house. I think Boggis couldn’t see it because he wasn’t part of the family and didn’t live in the house. I also think Tolly couldn’t see the children at the beginning and only hear them because, he was part of the family, and just moved in. Later he could see them because he lived in the house for longer, and know the secret.