This video provides tips for parents of autistic children in preparation for Halloween. It is also for those who will be giving out the candy on Halloween so they can be aware that some of the children who approach their door may be special needs.
5 Tips: Autism & Halloween (Hover)
I think most people will be understanding if you explain. Showing a card might be too much because you are putting them on the spot to read and respond. They may need their glasses or can't read or don't know much about autism.