No. Not at all. I didn't say that, either - I only said that adult status comes with responsibility. One way adults take responsibility is by paying bills. Of course, this is not always true. Eventually there will be an adult whom children should respect because he/she is an adult, even if that individual doesn't contribute financially. I'm thinking of my uncle, who had fallen into a rough spot in life and was living with his parents. Of course it would have been vastly inappropriate for myself and my cousins to treat him as our equal simply because he didn't have a job or a way to support himself - he was (is) still an adult.
I don't think at all that children's opinions don't matter simply because they pay no bills. I think it is perfectly acceptable for children to ask adults questions respectfully.
However, that was not the idea I got of the situation from what the OP wrote. Her daughter doesn't understand why she can't have adult status, "literally feels as though she should have the right to treat adults as peers" (to quote the OP,) and her questions are posed in a rude, disrespectful manner. I didn't get the feeling the OP felt she shouldn't have to respect her daughter. I have no doubt that the OP truly wants to help her daughter understand this problem in a gentle way.
But.
Granting adult status to a child is different from giving respect to a child.
The OP and her DH should be able to respect and value their daughter and her opinions without having to put up with an 8-yr-old treating them as her equals.
I gave that opinion in my pp simply because I felt it might be a good way to help the OP's child understand how different the adult's world is. Kids see all the "perks" of being adults, but sometimes they don't realize what some of the sacrifices are.
I'm sorry if my previous post was unnecessarily harsh.