If it’s written like 男の子 that just means boy, 100%.
People use 男の娘 in Japanese to achieve the “femboy” meaning you’re referring to. 娘 can sound the same as 子, but means daughter or young girl.
If people use “otokonoko” in English, there’s no way to distinguish the way the kanji does, but I imagine the only people using that in English are people who are only thinking of the second kanji form.