The miniature trains have Halloween themes--not specifically for SN kids:
http://www.montgomeryparks.org/enterprise/park_facilities/trains/documents/HalloweenTrains_2011_FLYER.pdf

Cabin John is spooky; Wheaton's is not.

If it's raining the trains don't run b/c the tracks can't be wet, so call ahead.

Basically for these kind of things, I just try to arrive early and not stay too long.

Also the Kennedy Center offers sensory performances for some kid friendly performances:
http://www.kennedy-center.org/accessibility/services/sensory.html

Forrest Hills playground (a few blocks north of the Van Ness metro) will do an event on October 26th from 10:00 am to noon. The whole playground is gated. There usually are crafts and a bake sale and a children's entertainer.

Four year old with Autism & ADHD. He is fine with crowds but will run away from me and is very hyper and his attention is everywhere. I'm new to the DC area so any suggestions or any "small" events would be helpful.