Make sure to expose your nonspeaking autistic kids to language.
Buy audiobooks related to their interests if you can find them, or get easy little picture books and read to them. Expose them to singalong videos where the lyrics are on the screen.
The goal is to expose them to words and what they mean. It doesn’t matter if they never use them. They hear or see you (sign language) using language.
Maybe one day your kiddo will bring their favorite storybook to you when you’re sad because it comforts them and they think it’ll comfort you, or they’ll point to a favorite picture in the book when they’re happy, or maybe they’ll fingerspell a song title or sign a lyric line from a song. Maybe, if they talk at all, it’ll be to repeat favorite sentences from the books or a lyric.
Even if they never speak spontaneously and only repeat stuff from the material you exposed them to, they have something they can use to communicate with you! Maybe they come to you and say, sign or type “Cinderella lost her glass slipper” because they can’t find their shoes. First off, that’s cute, and secondly they’re communicating with you in a way you can understand. That matters!
They might be really proud of themselves for being able to tell you something, don’t squash it by correcting them to say “I lost my shoes.”
Later on, they may start communicating with you using AAC and they’ll already have some building blocks for language laid down because you deliberately exposed them to it.
I won’t say everyone can do this, some people absorb language like a sponge and others may not. I’m not saying reading to your autistic kid is gonna magically make them able to communicate understandably with you (cuz mine-body disconnect is a thing for some), but it doesn’t hurt to give them a chance.
There are too many parents who assume their nonspeaking autistic kid doesn’t understand anything, so they don’t bother with language other than giving them commands. It’s really isolating to see everybody talking around you, but never talking TO you.
Language deprivation is a thing, just ask the Deaf community because they deal with it all the time.