I'm autistic, diagnosed in adulthood. I wish my parents had:
Found me alternate educational provision, or at least not made me feel terrible about the fact that I hated school and was too anxious to go. Instead I was told that I'd never get a job if I didn't attend school, among other things. Not true. The school years were absolutely the most miserable of my life. I self-injured regularly because I was so overwhelmed and unhappy, and still 'you have to go to school' was all I heard. I wasn't even allowed to try a different school.
Not been so dismissive of my special interests. Sharing them or feigning enthusiasm wasn't necessary, but telling me they were abnormal and a waste of time hurt. (I actually taught myself web design age 14 to make a website for a niche interest I had a lot of knowledge about. My parents just mocked it all.)
Not shouted so much. To this day I have an involuntary 'freeze' response if I'm shouted at, because as a child I knew that no matter what I said or did once it started, the yelling and name-calling would continue. I know nobody likes shouting, but the sensory overload made it worse for me.
Not lied to me whenever it seemed convenient because they thought that 'children don't believe their parents anyway'. No, to me they were my most trusted source of information about the world, so when they said things like 'if you move to another country when you're older we'll never see you again' or 'God knows everything you're thinking' - I believed it. Literally. (They weren't religious people who actually had that belief, but cultural Christians who liked using God for behavior management. Backfired when at age 8 I became incredibly anxious about needing to confess every 'bad thought' I had so I wouldn't go to hell...)
Been more careful with language. When I used to do things like stim I'd be told 'people don't do that' and my little autistic brain deduced that this meant I wasn't a person, because clearly I was doing it, and I liked doing it. From age 4 I used to fantasize that I was really an alien and one day my alien family would come and take me 'home' where people would be more like me. I really felt that isolated, and it's sad to look back on now.
My parents weren't bad people! And I don't think they knew how to handle the child they ended up with - very different to either of them. I'm old enough to have moved past all of this - but I do think my early adulthood would have been very different if I'd had more support and been valued for the person I was as a child.