Why do I use "sus" in this sentence?

In my spanish class I had to fill in the blank for possessive nouns. The sentence was:

¿Ustedes caminan con _________ hijos a la escuela?

I was trying to decide between tus and vuestros, and ultimately went with vuestros because the person asking could be talking to more than one person. Turns out the answer was sus, so I was wrong either way. Why is it sus in this instance and how can I tell in the future which to use?

submitted by /u/booksandwine99
[link] [comments]