I have two developmentally disabled students in my class - both adults, one with Cerebral Palsy, and the other with Downs' Syndrome. They both work very hard (the second more than the first), progress slowly, and absolutely love being part of the class.
The key to teaching students with developmental delays is to break everything - even little things that you think are simple - into the smallest possible steps. You need to keep the group small, and teach the things that you think are the most important for them to know. If you want to follow the standard syllabus, then you need to set up intermediates steps so that they can see their progress, which will be slower than students you are used to. For example, the two men that I have in my class test for intermediate belt ranks - instead of going from 10th gup to 9th gup, they tested for 1 stripe (9.5 gup, if you will), and then 9th gup, at two separate testings. One just tested last week for 6.5 gup - he was able to perform his tul with another person next to him, but not alone - when he can perform Do San alone, he'll test for the full green belt.
I accept a greater variation in technique from them; they both have coordination and fine motor problems, and are not able to perform techniques to the standard other students are held to - but they've both made great progress compared to where they started, and how many other people would stick with something when it takes so much longer? The man with CP started several months after another student who earned his BB in 5 years... and the man with CP is still a 7th gup; the BB has been gone for over a year, and the other man is still there.
I wrote my IV Dan thesis on teaching students with disabilities - ranging from short-term injuries to the man above with CP - I've attached it to this post.
Good luck, and let me know if you need more help/ideas.
