Great question! I’m a speech language pathologist and work regularly with children who have autism. (Also new to the community and posting for the first time to a forum, hope it works!)
Give yoga a try- more than one session too! The first time for anyone (doing really anything, including yoga) is new and people with autism need a little extra time and support to familiarize. I encourage foreshadowing what you’ll be doing by showing them a video of children their age doing yoga and use a visual daily schedule to let them know when they’ll do it and what they’ll do after (which could be a motivating thing or activity!). It’d be especially helpful to let them see pictures or videos of the body positioning, to bridge some receptive language gaps. I would also keep verbal instructions simple and concrete as you get into practice. If time boundaries are helpful for them to relax, offer a timer of some sort. A fun video of belly breathing could be a nice intro to start your practice as you tackle various yoga poses. Hope that helps!
Try new things or ways to engage their interests in the practice. If at first you don’t succeed , breath and hug yourself, and try again.