ds2 has dyspraxia and learnt the piano for a while. I think it is seen as a good - well in our case was - to help build up strength in the hands.
He quite liked it, but it was hard for him tbh, and in the end he decided to have singing lessons instead because really we needed to find something he liked and was successful at, because we have to practice so MANY other things, that the piano really wasn't one of them!
Key features - get them sitting straight at the piano, you may need to prompt the child to sit in middle of piano stool with feet hanging down. Playing both hands at once can be tricky, certainly worth concentrating on one hand at a time, and then using both. DS2 has a good sense of rhythm and this is helpful but finds it difficult to use both sides of his body at the same time - AND to sit straight in the middle of a chair too!
Break down your instructions and suggestions into small, bite size chunks. Then repeat. Don't overload with information. And don't think because he doesn't do what you asked, that they don't understand what you said - they do understand, but sometimes it's hard to sequence things properly and to ACT on the information which they understand intellectually.
Talk a lot - I mean verbalize things as well as demonstrate them - my ds2 finds this very helpful.
she may have AS and dyspraxia? They often go together - in which case other advice will be important too!