JimmyD,
I have been mentoring a friend photographer that also is catering to the autistic children market. She has been focusing her marketing efforts by working with support groups and understands that they can't take their children to "Wal-Mart" or "Sears" portrait studios because of thier childrens needs during a session, and that it is much more hands-on. You could have them do 'group' sittings, etc. and then word of mouth can spread into other segments of the community. Possibly donate proceeds from offshoot business back to the autistic community for free
PR.
Jim