Crossdressing boy
Though the normal usage "otoko no ko" is a young boy (child age), "Otoko no Ko" works because of the dual means that are allowed in kanji. Rather than explain in detail, I'll explain it with diagrams:
So it's simple as changing the "ko" from "child" to a "Ko" to a kanji that means "girl"
Can't figure out a good way to do this in English, so I'll just make the "ko" capitalized to "Ko" to make it distinct. If there's another method that works better, please provide an amendment.
Sample usage:
Kinoshita Hideyoshi from "Baka Test" and Bridget from "GUILTY GEAR" series are great examples of an Otoko no Ko.