A trans parent would likely still want to be called “Mom” or “Dad”, I assume. At least that’s the case with the few trans parents I know.
Parents don’t usually use “daughter” or “son” as pronouns, so I don’t think it would come up with non-binary children.
Do children of non-binary parents call their parents by their first name? It seems unlikely that they say “Parent, may I please have more screen time?”.
My niece is non-binary and we run into this issue. Now, I just ask people who identify as non-binary. “When there isn’t a good non-binary term to use, do you prefer me to use male or female words?”
So far, the non-binary people I’ve asked seem to appreciate the question.
I’m sure they do.
I think the gender inclusive term for niece and nephew is “nibling”, which sounds like a Pokemon!