In the last Harry Potter movie, why do the Malfoys "help" Harry Potter?

When Voldemort supposedly kills Harry in the Forbidden Forest, Draco's mother bends over the body and says "Is he alive Draco?" and Harry's head moves up and down confirming that he is truly alive. Also, Harry disarmed Draco of his wand in the room of lost things, therefore making the Eldest wand his (we find out later). Also at the end of the movie all three of the Malfoys leave Hogwarts together as Voldemort is being killed. I just don't understand how Harry moved his head when his mother asked Draco (who wasn't even there) if he Harry was alive or not. If anyone could explain why the Malfoys suddenly wanted Harry to live I would be thankful.