Jojo is a 10 years old boy fanatic nazi, who's best friend is Adolf Hitler himself. That says a movie description and Jojo's mother. But we didn't see it at all. Jojo is just a kid, who likes swastikas and the idea of being nazi, but he never feels like one. You never "buy" it, because Jojo is so cute and fails in his first "mission" to kill a rabbit. In fact, all the Deutsches Jungvolk looks more like a scouts to me (maybe because of English language and american maniers). And because of it, we never feel a real fear and atrocities of Nazi ideology and war itself. Maybe the movie's idea was to show us that kids were just a kids even in Nazi Germany, I don't know, but I think the movie would be better if it's tone were more serious.

Should we use homour anyway? The themes like WW2 and the Holocaust are extremly serious. The humour could be used to show it's absurdity and insanity. The parody, sarcasm and irony are good tools to show it. But it's very difficult because the topic is so sensitive. Probably the best example of movie that succeed in it is R. Benigni's "Life is Beautiful". Also I would recommend to read amazing book "The Forest of the Gods" by Lithuanian author Balys Sruoga.