Pickles asked for some insight since seeing in another post that I have an autistic child. I'm more than happy to share!
My daughter is smack in the middle of the "spectrum" and is in a "cross categorical" class. One of her classmates is more severely autistic, and the others are high functioning. I've been around them for a couple field trips, to read a story, and just in general.
I'm trying to imagine how it would go to have a clown come in. I think most of them would enjoy it, and a couple might get anxious. There could be some keening, crying, or even screaming from kids who don't tolerate change/new events (I'm thinking of the one boy in particular). If this happens, reasonable adults-in-charge will take an upset child to a quiet room. It's not a punishment or bad thing. It is what it is.
For the most part, and I saw this mentioned in the other posts, many of these kids are VERY tactile. My daughter is one of them. They can be driven to touch new and interesting things, especially if there's obvious texture and wild color (in other words, a clown!!!!). Maybe you can bring little "sensory" toys you can hand out if the kids get too touchy-feely. If latex isn't an issue, those koosh balls are always a hit. My daughter also loves pin wheels, and you can get bunches of them cheap at Hobby Lobby or the like. Distractions like that are great--as long as you bring enough for everyone. Of course, you still risk losing their attention, so it's a balancing act.
Ask the parents about balloons. Also, make sure none of the kids has a latex allergy. It seems like there are more problems with allergies among special-needs kids than "regular" (I say "regular" with a block of salt) kids. I just had the idea last night of using non-latex balloons and putting some drawings with the child's name on the balloon. Since I'm allergic to latex, I'm trying to think of alternatives, and that's all I have so far on that count.
Face painting is tricky. My daughter has a horrid time sitting still, yet her friend let me paint a half face with bowling, and he held perfectly still. This blew me away because he also had ADHD, and I've NEVER seen this kid hold still before!
The biggest thing to remember is to BE FLEXIBLE--figuratively speaking, but it would be
really funny if you could be literally flexible!
There is no predictability, even if you're around these kids on a daily basis. Believe me, my daughter can be a different person in a flash. This isn't always a bad thing, but you have to be prepared to go with it.
Try to see if there will be music available. My daughter especially loves Laurie Berkner and Dan Zanes. I HIGHLY recommend having at least one LB and/or DZ album in your collection if you are one to have music on hand. Check out "We Are the Dinosaurs" and "Victor Vitto"--most kids LOVE these songs, especially the cross-cat kids I know.
Finally, try to get an idea of the functioning level of the kids, and of some things they like. Do they love certain characters? Sports? Stories? If you can tie in something that they individually or as a group like, then this will also help.
Kudos, and I mean HUGE, BIG, WONDERFUL
KUDOS to you for taking the time to prepare. I hope it goes well. Even if you run into bumpy moments, it'll be an experience you'll take to heart.
As I learn more about clowning (I really am a newbie!), I'll update what I've said. My only expertise is from being a mom and being around Zoe's class and friends.
Have a great time!
Christina