Because "boys/garçons" is a plural, so it needs that the possessive adjective is also plural: VOS.
Remember that in French, the possessive adjective does not agree with the owner but with the object.
"Vous" is the formal version of "you", like "tu" is the informal version. "Vos" is the plural possessive, meaning "your", when talking about multiple things.
Nowadays, you can read "les nouvelles" on your laptop or on your smartphone, so "les nouvelles" cannot be synonymous with "newspaper".
Duo does not expect learners to adapt the sentences proposed, but to translate them as close as possible to the original version, whenever it is possible.