This was discussed here:
Not so, at least here in the UK, where we talk about Christmas presents, birthday presents, etc far more than "Christmas gift" or "birthday gift". Do we really view Christmas and birthday presents as rewards? "A present for my daughter" was accepted as correct for me here.
In almost all cases, nouns that end in o are masculine and words that end in a are feminine. There are some exceptions and irregularities, like “el agua,” but you can remember a majority of them because they’re like that because it would sound odd otherwise and be hard to pronounce quickly.
As a native Spanish speaker I would say that although both "mija" and "mijo" are sometimes used in particular regional and/or social contexts, its use is not nearly as widespread as the standard "mi hija" and "mi hijo", respectively. They are mostly used by elderly people when addressing younger people (not necessarily daughters or sons) in an informal and paternal way. In fact the pronunciation is not exactly the same, even if the difference is subtle, and its use is almost purely verbal; you will rarely see them in written language. I cannot imagine a situation in which anyone would write "mija" in this sentence. For all those reasons, even if "mija" could be technically correct, I absolutely support Duolingo in its rejection as a valid alternative.