"No, thanks, boys."
After a second thought: I guess 'Jungen' doesn't work because the sentence is a direct address. It sounds very aloof if you directly address a group of boys as 'Jungen'. If you speak about a group of boys, 'Jungen' is generally preferable. I'd say 'Die Jungen gehen spielen' but always 'Jungs, geht mal spielen!'.