why the les?
In French, generalities use the definite article.
what is a generality?
it is when you mean something collective, valid for a whole group of people or things, like "men are strong", which means "in general, men are strong".
Why can't you use des instead of les? I mean there's no the before the boys?
I THINK it's because you're referring to the entirety of the group of boys you are addressing, and not just some of them. Could be totally wrong though.
Even if you're talking to the boys, you'd still say LES garcons?
THE = LES