I'm autistic and I work with lots of kids with autism. Another thing to consider is that a lot of autistic children require aides at school that work closely with them. To protect these aides, it's important to help kids learn to tolerate masks and masks will be at most schools this year. ADA does not apply to things that are for public safety. I've found some really sensory friendly cloth masks a friend is making me. It's been totally fine. Just takes some getting used to. Sensory needs for people with autism vary, but we have all learned to tolerate pants, shirts, and at least some underclothes. Masks are new and different, and it will take time to adjust. We need to be gentle and sensitive to sensory needs as we approach this. Try lots of different types of masks. Some tie, some have elastic, they have different materials, etc. Florida heat would make this harder for people like me who are already heat intolerant and bothered by the feeling of just sweat. So to protect all of you wonderful people, since I really do not want to wear a mask in florida heat, I'll stay home until the fireworks come back. Winter would be fine for a mask and i totally may do it.
Also, if your autistic child wears noise cancelling headphones in the park, practice wearing these with a mask before you go. It feels different! I do mask, then headphones. Also, an adult masks might be able to go over child sized headphones instead of ears. Still does the job and elastic does not touch ear!
Be creative and we can help people with autism be safe and accommodate sensory needs. Just like you can't forego pants in the park due to sensory needs and parents have to find what works. Same thing here. Short practice sessions, trying lots of different things, and listen to the autistic person about what they prefer will be key.