This year, the vegan/vegetarian family group which I lead, Vegkins, had a Halloween party. We decided to make it a potluck to take the burden off of organizers to prepare and purchase all the food. That worked out really well using evite's potluck option, so everyone could select a variety of dishes to bring.
The Animal Rights Coalition graciously paid for the room rental at a local park's recreational room. A couple parents chipped in to pay for the rest of the items (prizes, dinner ware, crafts, and treats). Overall it was a big hit.
The turn out was really great, and the families had a nice time. If you are interested in planning a vegan Halloween party in your city, I recommend contacting local animal rights groups for help getting the word out and organizing.
Considering the success of this year's party, I know we'll be having another one next year.