Is "Jungs" a relatively new (post WW2) word in German? I have heard German spoken from my my parents on occasion since a child, from my relatives when I have visited Germany and traveled in Switzerland and Austria but have never heard the word "Jungen".
I have heard "Buben" used among my relatives for boys. But again almost all of these people were born before WW2 in the area east of Heidelberg, along the Neckar River and west of Eberbach.
It's actually der Junge / die Jungen. I'm not sure how much it's actually used in Germany. It was not in any of the books I used to learn German. I suspect that duolingo wanted to have a word for "boys" and decided to use this one, for lack of a better one.
"Jungs" is a bit informal and used pretty commonly, not only for children but also for sports teams etc. "Buben" is a bit outdated, but still in use by elderly people in south germany. "Jungen" is used pretty seldom for boys younger than about 10 years. In official publications your most probably will find a formulation like "Jugendliche" or "männliche Jugendliche" (for boys >10 years).