S.H.
Okay, she is 6.
Have you ever just told her that whining/her tone of voice is just icky?
Record her. Let her hear it.
Then just talk to her and tell her plainly.
It is a real UNpleasant way of talking.
Sometimes you just gotta be point blank and straight forward. Not trying to over explain things.
A 6 year old KNOWS what is not nice or whiny. They KNOW.
She is 6. Just tell her.
I have a 6 year old, when/if he uses an irritating voice, I just tell him.
No sense beating around the bush or hinting.
And then he laughs and realizes he does sound icky in tone.
Or I imitate him and tell him "this is the way you sound... what if Mommy sounded like that ALL day. Not fun eh?"
And.he.gets.the.point.
I work at my kids' school, I see kids this age all the time and deal with them. If they talk like that, I tell them that is not the way to talk. Try again... SHOW me you know how. And they DO know how.