Querían means it is something they used to do in the past. But Quisieron is a specific instance.
So for querían, you could be talking about how your parents were over an extended period, ie "My parents used to want to drink wine when I was growing up, but now they only want soda."
But if you needed to say, "We were at a restaurant last night and my parents wanted to drink wine" you would use Quisieron.
I agree about 90% with what you said. I agree that the preterite can be used for the one time occurrence. However, it is not necessary. With a verb like querer, the imperfect is almost always OK in the past, because it describes something, it's not really an action.
Furthermore, I've been taught that querer in the preterite usually implies that the desire was fulfilled...the thing that was wanted actually took place. In the case of the wine, the preterite would be more likely if the parents wanted to drink wine and they did. If would not be nearly as likely to use it in the preterite if they wanted to drink wine, but the restaurante didn't serve it. This makes sense. They probably continued to want to drink wine, so the imperfect conveys the sense of this desire not being completed.
Yes, it looks as if they meant to have
"Mis padres querrían beber vino" (My parents would like to drink wine)
but somehow it ended up as the imperfect ("querían") instead, and they just left it. (My parents wanted to drink wine.)
Mind you, it brought home to me how similar those two conjugations can sound.
"Padres" can mean "parents" or "fathers." Like almost everything in Spanish, the masculine is default, and plural masculine could mean multiple males, but it could also mean a mixed group that has a male in it. I guess Spanish is inclusive like that (except for females whoops)?
((if anyone does know anything about gender neutral pronouns and how to use them in Spanish, send some links! i've been wondering about that))