"Your boy is swimming."
Translation:Jullie jongen zwemt.
That would need be "jouw jongen" because it's posesive however it would turn the posesive party singular
It's also possible to use je
That would make it slightly more casual and singular
'Jullie' is plural and 'jouw' is singular, that is, as possessive pronoun:
Zij nemen jullie boeken (they take your books (plural))
Hij neemt jouw boek (he takes your book (singular))
What makes it complicated is that 'jullie' is also used as a personal pronoun:
Jullie nemen een koekje Waarom zitten jullie daar
You can combine them of course:
Jullie nemen jullie boeken (you take your books (both plural))