This works best for a girl....no sewing involved. Go to a charity shop and buy a smallish womans long black dress. Cut off the bottom in a deep zig zag pattern so that it's longish on the child. Split the back of the cut off part of the skirt down the back seam to make an oblong with the old hem at the top and zig zags at the bottom. Thread a long piece of tape through the hem, gathering it to make a short cloak with a zig zag edging which matches the dress. The old hemline is now the neckline of the cloak and ties up with the tape around the neck. Get some fabric and cut out moons and stars and iron them onto the dress and the cloak using double sided Vilene. You can get this from anywher that sells cotton, zips etc. Don't try to make a witch's hat, buy one at almost any sweetshop or Woolworths. Good luck.
Chicken wire + orange crepe paper or papier machier painted orange = pumpkin outfit. The downfall is that if your child is prone to falling over, you may have to chase after them to stop them rolling away! But it looks gooood.
As an alternative to whiskygalore make the globe small enough to fit around the head with some space, use white cloth or strong white paper to make an oblong sack that'll drape from the shoulder and paint black stripes across it. Get a small torch with a flashing bulb, tie on top of head, switch on and you've got a Zebra Crossing - great if you're child wants to be secretive, if quiet no one will know who it is