The offer is only available in-store at Blacks, on Friday the 30th and Saturday the 31st of October 2009. You must bring your child in, dressed up in their Halloween costume. The Black's staff will then take a photo, and print it out in a Halloween themed setting.
They're probably doing this to try and get you in to buy a camera or something, so I'm sure you will hear a sale pitch when you get there. But there is no obligation to buy, so just go claim your freebie, and don't feel pressurized.
Of course, if you really are looking for a new camera, this could be a good opportunity to test the waters.