Як я розумію, молодший це той що найменший саме про сина. То чому ж тоді younger а не youngest. Коли менший брат, то не відомо чи він найменший. А у матері один молодший син.
So if Molodshy (sorry that's probably not spelled right :P) is the male form and Molodsha is the female form, is there a plural form? (like: My younger sisters/ brothers?)
it gonna be 'Molodshi' (Moi molodshi brati - my younger brothers) - for male, female (universal form )
I think the "youngest" shouldn't be marked as "wrong". It all depends on context. This phrase doesn't have a context.
While it is possible that the younger son is also the youngest son, it is not the emphasis of this sentence. If the writer had wanted to say the word "youngest" he would have added another word to show that he was the "most younger". IMHO, the sentence stands on its own with or without an expanded context.
In the course English for Ukrainian-speaking an article THE was translated as цей-ця-ці (equally to THIS-THESE). Perhaps, the same principle is used here, since the developers are the same.
It's possibly that it was marked incorrect because you used "the". The two words are not being used in a sentence and only a translation is asked for. It appears it would be different, and correct, if "the" was needed in an english sentence for it to be grammatically correct.