Rather than one term, then, the translation should use two, “lad” and “lass”, in singular, in plural, and in combination.
The problem is that “lass” doesn't really sound that way in the same authoritative fashion. Addressing someone with “lad” carries a bit of a connotation of being in charge and knowing better, which “lass” doesn't. It also sounds very Scottish and “lad” doesn't necessarily though Scotsmen are also known to use it.
I don't necessarily disagree with your objection in any case. Translating from Japanese is a constant judgement call between tone and gender. There are many other things such as many translators opting to call female characters “the daemon king” in a translation. Obviously that is highly unusual in English but not unheard of either with Elizabeth Swan being referred to as “The Pirate King” in
Pirates of The Caribbean. The translator didn't want to gender the Japanese term but some translators may choose to do so. I could see another translator use “lass” and I wouldn't fault it but I chose tone over gender here.