Learn Dutch: Kennen jullie zijn naam?

It does! Zijn is the verb "to be", but it is also the word for "his". You can tell which one it is from the context. Because it comes directly before the noun naam (name) in this sentence, it is acting as a possessive word meaning "his".

Why do we use kennen here instead of weten for "to know"?

Dutch has two verbs for "to know". Weten is used for knowing facts or information. Kennen is used for being familiar with a noun, like a person, a place, or a thing. A name (naam) is treated as a noun you are familiar with, so we use kennen.

Why does the sentence start with kennen?

To form a yes/no question in Dutch, you swap the verb and the subject. A normal statement is Jullie kennen zijn naam (You know his name). By moving the verb to the front, it becomes the question Kennen jullie zijn naam?

Do Dutch speakers sometimes pronounce zijn differently?

Yes! In casual speech, the possessive word zijn is very often shortened to z'n, which sounds a bit like "zuhn". So in real life, you will frequently hear this sentence pronounced as "Kennen jullie z'n naam?"