I have to wonder how this turned out....
In 2006, my son with autism (not high or low functioning, just autism) was 10. He was very hyper and unfocused, but loved the guitar and banjo. He had toy versions of each and would give "concerts," pretending to play while he sang songs he knew. We encouraged him and joined his band when he would let us. It was an opportunity to co-regulate and do something together.
If I were starting with a student at this level, I would just have them play the 6th string along with me and sing "E." I might play songs and let them "play" with me.
Fast forward to 2009. My son (now 12) became seriously interested in playing the guitar. We gave him short - ten to fifteen minute - lessons. He watched videos on how to play guitar. We slowed it way down and taught him chord shapes. He began to play. He began to watch other guitarists and musicians.
Along the way, he became interested in Irish music and began to study banjo, then pennywhistle, then mandolin. In 2014, he joined an Irish band as a paid member and did gigs around Los Angeles. He is not a savant; he is someone who works very hard at his craft.
Not saying all kids (even neurotypical kids) would have the same story. But what most people enjoy about music is doing something together. Most young kids aren't looking to be virtuosos, but just to be able to play simple 1-4-5 chords and sing along with their friends.
Think of kids with autism as kids who have trouble communicating but have the same interests, dreams, and opinions as other people. Some, like my son, will love music. He works hard at it and practices for hours every day. Others will pass through your studio just as other 10, 16, or 20 year-olds do. It will be something they try. Make it fun and make it relational and they will be better people for it even if they never pick up an instrument again.