"Why do you want to say that to my mother?"
Translation:Kial vi volas diri tion al mia patrino?
I have no idea, but I had the same solution. The word order is somewhat unnatural in English.
Why you want to say to my mother that? vs. Why you want to say that to my mother?
Personally, I like the first one because it clearly indicates that you're saying that thing to my mother. "diri al mia patrino" feels like a phrase that should stick together.
I think the takeaway here is that both work and would be comprehensible.
Plus, there's this other question: "Ne, mi ne volas diri al vi mian nomon." Which follows precisely the OTHER word order.
The accusative -n here indicates what is being said (tion) and to who so it doesn't really matter I think? So long as you're using the ki- word at the start, remembering your accusative -n and keeping "al mia patrino" together it shouldn't matter about the rest.
Both answers should (AFAIK) be accepted and if they aren't, use the little "Report -> My answer should be accepted" option to help improve the course.