I have two young sons with high functioning autism and have worked with others. A lot of these kids have sensory issues. Sometimes if their "autism" gets in the way of learning, it is good to get them away from the piano and have them do a physical activity to help them regulate or regroup.
For example, My 4 yo can't sit for too long to work on a project that requires a lot of fine motor skills. He will often be put on an exercise ball and be bounced for a few minutes. Then he will be able to go back and finish a task. It sounds silly but these kids often have lots of "quirks" that the rest of us don't understand.
Kids with ASD are usually strong visual learners and have kean auditory skills when it comes to music. Also autistics tend to be very musical but at the same time have auditory processing issues where the "wiring from their brain to their ears and mouths is not working properly.

My older son (6yo) takes piano lessons. One of the things that helps is to point and sing the notes before he actually sits down to play the song. I know some teachers think this is useless but it helps ASD kids to see the music and hear the notes. At the same time it becomes rote and in his "quirky" way he learns to recognize the notes. My son is great at reading the notes on the grand staff but when we look at flash cards of just one staff and the note, he can't name it.

Anyway, as a parent of two kids with asd, I can tell you that it means a lot to the parents that you're teaching their kids. Most people would not want to attempt to even try.

Anyway it sounds like you're on the right track....Just keep it fun. Keep your sense of humor. Don't expect less of these students, just remember it might take longer to get there.
Good Luck