Kensington Sands had big plans for Halloween. She was nine years old now, and bored with dressing up. She wanted proper thrills and frights.   "Grandpa Jonas, can you come over tonight? I've cancelled trick-or-treating but only because I've got an even better idea. Something truly scary. You'll see!" "Tonsil-wobbling terror? I'll be there!" Grandpa Jonas promised.   Kensington spent that afternoon making ham-and-pea soup. When it was done, she took up the old polaroid camera she'd been given for her birthday. "Hold still now," she told Arantxa, her Spanish Water Dog. "Look!"   Kensingt