How about boy detective?
...How about it? Never heard this use before, and I'm pretty traveled.
There is one other option I didn't mention, and it's to leave the word Shounen in the title. Shounen Ninja Sasuke. We all know what Shounen is from Shounen Jump, or maybe we don't. I think many, many people do because of this popular manga magazine.
It's weird that you call mail man an exception, since it's formatted in the same way you think this title should be. It would make more sense to include in your argument as an exception if the term was "man mail".
No, it's not weird in the slightest. In fact it's alliteration and that is the only reason the expression mail man exists; but I explained this to you already.
Girls are called kids all the time, and not just in groups.
Yeah, sure, all the time. It's just that I've never heard a solitary girl being called a kid, when in this specific instance it's more of a synonym for a tough, a gang member, a ruffian, etc. If you don't want to change the title, don't. I'm not the one who's doing the translation. However, when you ask somebody who's been taught the English, then he's going to give you his pointers, whether you take them or not.