Grace and the kids see everything in the house as it was when they were alive, with their possessions all where they were at the time; when we later see the glimpse of the living world with the family and the medium in the kids' former bedroom, the kids possessions are no longer there, but they are again when the film shifts back to the perspective of the dead. With that in mind, why were they able to perceive the curtains being pulled down? Everything that was in the house when they were alive has already been removed without them "noticing", so why did they notice the curtains?
Mrs. Mills tells Grace at the end of the movie that 'sometimes we'll be aware of them, sometimes we won't' (in regards to 'intruders'). Grace and her children were clearly picking up on Victor and his family, and pushing back enough that Victor and his family were aware of them. The living then upped the ante by hiring a medium to make contact, which presumably broke the lines between the world of the living and the world of the dead even further, making the ghosts able to perceive some of the changes going on.
