Is Ustedes plural. If so Ustedes quieren: they want, would be correct.

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Hola Carl
I think you are confusing the pronoun ustedes. Ustedes is not "they", it is "you" (plural), used formally in Spain and formally and informally in Latin America. 
Here is a list with all subject pronouns.
Saludos

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Hola!

From my understanding, It wouldn't because Ustedes is plural but used when you are directly speaking to the people ,

It wouldn't be correct to say "Do they want ice cream" instead of "Do you want ice cream ?" Just because you are asking a lot of people right?

Sorry for my bad explanation, hope this helps anyways .