"You" is actually already the second person plural in English. It was also the polite form for the singular, and eventually displaced it in most English speech. The singular in English for hundreds of years was "thou" as in "How great thou art." The direct object was "thee": "I do wish thou wert a dog, that I might love thee something...." The possessive was "thine", as in "drink to me only with thine eyes." There are areas of Britain where the second person singular is still used, and it was commonly used by many American Quakers and Anabaptists (Amish, etc.).
According to Wikipedia - if I remember correctly it's under "Dutch grammar" - it seems that long ago the Dutch had something similar. As happened in English it fell out of use, so that like we English-speakers they were using the same word for singular and plural "you". Unlike us they dealt with it. What they did was change the plural "you" along the lines of "y'all". That's how "jullie" came about.
In German "jung"/"junge" is the adjective "young" and "Junge" is "boy". The two languages are alike at times. In Dutch "jongen" is "boy", plural "jongens", of course. Thanks, Christian81, for telling us that in Dutch young is "jong"/"jonge". So alike!
I'm not sure if that's a good or bad thing for learners like me who know some German. You'd think it would encourage me but oddly enough it sometimes puts me off! I suppose I have a sense of "Oh, I've already learned that and now I'm having to learn a variant of it!" (No disrespect to the Dutch language here. It could have been Dutch I tackled first and then German would seem a variant of it!)
You are boys shouldn't be said like that. You are boys. It should be something like You guys are boys. You = 1 so they're talking to one person saying they are multiple boys. You guys = more than one person is being talked to, which means they are saying all of them are boys.