The term otoko-no-ko came about first. Femboy, while not exactly equivalent, was pushed as a 'less offensive' (just going to set aside the entire is-it-or-not debate here) english term for otoko-no-ko than trap. Otoko-no-ko itself is a pun on otokonoko (boy), so in japanese the whole point is that you can't tell, even in speech, whether you're referring to a boy or not. It's very elegant in a silly way.
Otoko no musume is, literally, 'boy daughter'. Not in the way otoko-no-ko gets there, through some ambiguous kanji readings, but directly and bluntly.