There are two types of plural in Polish. One is "masculine personal", and the other "not masculine-personal". The first one describes groups of people containing at least one male, and the second is for everything else. As "chłopcy" are definitely all male, it is masculine personal. And the right form is "wasi" (or "twoi").
Generally, "chłopcy" are younger and "chłopaki" are older. It's hard to set a border when one changes into the other.
Logically, "boys" are masculine personal, so the form of "your" should be "twoi". Like in "twoi chłopcy". But because of an unknown reason, in Nominative, "chłopaki" are not masculine-personal: they take not masculine-personal pronouns, adjectives, verbs (it would be visible in Past Tense). That's why it's "twoje chłopaki". In all other cases they behave normally like a masculine personal noun.
Never mind.. read the comments, not sure my old polish teacher would have agreed but this is how it is here... thanks