It's dative. Plurals in dative case use "den".
Have a look at this link: http://german.about.com/library/weekly/aa033098.htm
"Wie gehen die Jungen?" means "how are the boys going" in the literal sense of "how are the boys getting there?"
"Wie geht es [dat]?" is the construction for "How are/is [person]?" Also, it's a deeper question than a casual "how are you?" in English. It is not something that you would ask people you see everyday.