Souplantation and Sweet Tomatoes: costumed kids eat free (one per grown-up purchase) on Friday and Saturday 30/31. Info here.
Taco Bell: free Black Jack Taco per person from 6pm - midnight. Details here.
Papa Murphy's Pizza: each constumed kid can get a personal-sized pizza! This is supposed to be nationwide, but some of the stores are franchised, so I would definitely call ahead. Check out the store locator here.
Also - don't forget the round-up of fast food deals and booklets! And free drinks at 7-eleven until the 31st.