"The boys eat at the restaurant."
Translation:Die Jungen essen im Restaurant.
"Jungs" is colloquial plural form of "Junge" in some parts of Germany, while "Jungen" if formal plural form. Perhaps Duolingo only accepts formal form.
The declined forms of "Junge", as being a "weak" noun, are "Jungen" in accusative, dative, genitive singular as well as in plural in all cases. (http://www.canoo.net/inflection/junge:N:M:boy )