We did this years ago when Molly was 7.
We did a sweetie bar. I had gummy worms as jellied slugs. Raspberry laces as gillyweed. Freddie for chocolate frogs.
We made witches hats and wands from cardboard and wooden dowels and they decorated them with foam sticks stickers and glitter.
Magic drinks (put a tiny bit of food colouring under the ice. Then pour in clear lemonade )
Pin the tail on the dragon
Probably our best themes party ever and pretty easy. Especially if you pick things up around Halloween.